Xml export of an excel document fails if it contains empty cells which are mapped to an xml element of type double and attribute minOccurs=0. Poi fails to export such documents because it fills in an empty string into these xml tags. This leads to a schema validation exception in case of an element with a double datatype. The patch I'm going to provide handles it the same way as Excel does. It removes empty elements from the output xml.
Created attachment 31145 [details] [PATCH] Patch
Thanks a lot for all these patches, this one is now applied as well via r1564045.