HTML export filtering doesn't work when filter=installedExtensionDocument is specified

To reproduce: execute the following URL: http://localhost:8080/xwiki/bin/export/Space/Page?format=html&pages=%25.%25&name=AllPages&filter=installedExtensionDocument

What happens is that getNodeReference(} will return xwiki:%.WebHome and getChildren() will return an empty set since it's trying to find all docs under that node but it contains a % which is not considered a wildcard and thus it won't match anything.

cc mflorea who implemented this code.

Also, the filter parameter is not documented at https://www.xwiki.org/xwiki/bin/view/Documentation/UserGuide/Features/Exports#HHTMLExport