PDF export breaks for content using Soft hyphen

Steps to reproduce:

• Create a new wiki page
• Add the following content in the editor (which includes a soft-hypen with character code 173 at the beginning of the string):

  ­    Test
  

• Save the page
• Export the page as PDF
  Result: the PDF document cannot be generated and the logs display an error

  Caused by: com.xpn.xwiki.XWikiException: Error number 12002 in 12: Exception while exporting pdf
  	at com.xpn.xwiki.pdf.impl.PdfExportImpl.createException(PdfExportImpl.java:569) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.pdf.impl.PdfExportImpl.renderXSLFO(PdfExportImpl.java:343) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.pdf.impl.PdfExportImpl.exportXHTML(PdfExportImpl.java:271) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.pdf.impl.PdfExportImpl.exportHtml(PdfExportImpl.java:224) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.pdf.impl.PdfExportImpl.export(PdfExportImpl.java:210) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.web.ExportAction.export(ExportAction.java:223) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	at com.xpn.xwiki.web.ExportAction.render(ExportAction.java:79) ~[xwiki-platform-legacy-oldcore-5.4.6-20140723.153439-12.jar:na]
  	... 44 common frames omitted
  Caused by: java.lang.IllegalArgumentException: factor < 0; was: -1
  	at org.apache.fop.traits.MinOptMax.mult(MinOptMax.java:251) ~[fop-1.0.jar:na]
  	at org.apache.fop.layoutmgr.inline.TextLayoutManager.processWord(TextLayoutManager.java:825) ~[fop-1.0.jar:na]
  	at org.apache.fop.layoutmgr.inline.TextLayoutManager.getNextKnuthElements(TextLayoutManager.java:663) ~[fop-1.0.jar:na]