Class pages exported on newer XWiki versions fail to be imported on older instances

XWIKI-7783 introduced a new class meta-property, hint, which seem to cause the import (both manual and through Distribution Wizard) to fail on older instances (<XWiki 9.11-rc-1).

• The manual import just shows the page in the "Erroneous pages" section. No info in he logs.
• Distribution Wizard seems to completely skip the page from the UI log. Opening the jobs status xml logs, a NPE seem to be thown (since there is nothing resolved in the currentClassPropertyMeta map, in the import code). Also note that the failing document is no specified in the exception, which makes it harder to know what fails:

        <org.xwiki.logging.event.LogEvent>
          <level>ERROR</level>
          <timeStamp>1518798355093</timeStamp>
          <message>Failed to parse document</message>
          <throwable class="org.xwiki.filter.FilterException">
            <detailMessage>Failed to read XAR XML document</detailMessage>
            <cause class="java.lang.NullPointerException">
              <stackTrace>
                <trace>org.xwiki.filter.instance.internal.output.XWikiDocumentOutputFilterStream.onWikiClassPropertyField(XWikiDocumentOutputFilterStream.java:573)</trace>
                <trace>sun.reflect.GeneratedMethodAccessor434.invoke(Unknown Source)</trace>
                <trace>sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)</trace>
                <trace>java.lang.reflect.Method.invoke(Method.java:498)</trace>
                <trace>org.xwiki.filter.internal.FilterProxy.invoke(FilterProxy.java:90)</trace>
                <trace>org.xwiki.filter.internal.FilterProxy.invoke(FilterProxy.java:63)</trace>
                <trace>com.sun.proxy.$Proxy79.onWikiClassPropertyField(Unknown Source)</trace>
                <trace>org.xwiki.filter.xar.internal.input.ClassPropertyReader$WikiClassProperty.send(ClassPropertyReader.java:57)</trace>
                <trace>org.xwiki.filter.xar.internal.input.ClassReader$WikiClass.send(ClassReader.java:61)</trace>
                <trace>org.xwiki.filter.xar.internal.input.WikiObjectReader$WikiObject.send(WikiObjectReader.java:85)</trace>
                <trace>org.xwiki.filter.xar.internal.input.DocumentLocaleReader.sendWikiObjects(DocumentLocaleReader.java:522)</trace>
                <trace>org.xwiki.filter.xar.internal.input.DocumentLocaleReader.readDocument(DocumentLocaleReader.java:467)</trace>
                <trace>org.xwiki.filter.xar.internal.input.DocumentLocaleReader.read(DocumentLocaleReader.java:352)</trace>
                <trace>org.xwiki.filter.xar.internal.input.DocumentLocaleReader.read(DocumentLocaleReader.java:325)</trace>
                <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.readDocument(XARInputFilterStream.java:120)</trace>
                <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.read(XARInputFilterStream.java:70)</trace>
                <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.read(XARInputFilterStream.java:47)</trace>
                <trace>org.xwiki.filter.input.AbstractBeanInputFilterStream.read(AbstractBeanInputFilterStream.java:79)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.getXWikiDocument(Packager.java:343)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importDocumentToWiki(Packager.java:199)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXARToWiki(Packager.java:175)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXARToWiki(Packager.java:146)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXAR(Packager.java:136)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.XarExtensionHandler.installInternal(XarExtensionHandler.java:203)</trace>
                <trace>org.xwiki.extension.xar.internal.handler.XarExtensionHandler.install(XarExtensionHandler.java:167)</trace>
                <trace>org.xwiki.extension.handler.internal.DefaultExtensionHandlerManager.install(DefaultExtensionHandlerManager.java:85)</trace>
                <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.installExtension(AbstractExtensionJob.java:268)</trace>
                <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.applyAction(AbstractExtensionJob.java:201)</trace>
                <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.applyActions(AbstractExtensionJob.java:151)</trace>
                <trace>org.xwiki.extension.job.internal.InstallJob.runInternal(InstallJob.java:146)</trace>
                <trace>org.xwiki.job.AbstractJob.runInContext(AbstractJob.java:206)</trace>
                <trace>org.xwiki.job.AbstractJob.run(AbstractJob.java:189)</trace>
                <trace>java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)</trace>
                <trace>java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)</trace>
                <trace>java.lang.Thread.run(Thread.java:748)</trace>
              </stackTrace>
            </cause>
            <stackTrace>
              <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.readDocument(XARInputFilterStream.java:122)</trace>
              <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.read(XARInputFilterStream.java:70)</trace>
              <trace>org.xwiki.filter.xar.internal.input.XARInputFilterStream.read(XARInputFilterStream.java:47)</trace>
              <trace>org.xwiki.filter.input.AbstractBeanInputFilterStream.read(AbstractBeanInputFilterStream.java:79)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.getXWikiDocument(Packager.java:343)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importDocumentToWiki(Packager.java:199)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXARToWiki(Packager.java:175)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXARToWiki(Packager.java:146)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.packager.Packager.importXAR(Packager.java:136)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.XarExtensionHandler.installInternal(XarExtensionHandler.java:203)</trace>
              <trace>org.xwiki.extension.xar.internal.handler.XarExtensionHandler.install(XarExtensionHandler.java:167)</trace>
              <trace>org.xwiki.extension.handler.internal.DefaultExtensionHandlerManager.install(DefaultExtensionHandlerManager.java:85)</trace>
              <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.installExtension(AbstractExtensionJob.java:268)</trace>
              <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.applyAction(AbstractExtensionJob.java:201)</trace>
              <trace>org.xwiki.extension.job.internal.AbstractExtensionJob.applyActions(AbstractExtensionJob.java:151)</trace>
              <trace>org.xwiki.extension.job.internal.InstallJob.runInternal(InstallJob.java:146)</trace>
              <trace>org.xwiki.job.AbstractJob.runInContext(AbstractJob.java:206)</trace>
              <trace>org.xwiki.job.AbstractJob.run(AbstractJob.java:189)</trace>
              <trace>java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)</trace>
              <trace>java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)</trace>
              <trace>java.lang.Thread.run(Thread.java:748)</trace>
            </stackTrace>
          </throwable>
        </org.xwiki.logging.event.LogEvent>