Hi,
If you have defined your XML schema element as minOccurs 0 and it has simple content them then is should not be exported if the value is null.
This process is described in John Brinkman's blog http://blogs.adobe.com/formfeed/2009/12/null_data_handling.html
If you element has complex content (attributes or child elements) then you will have to edit the data description in the XML Source view of Designer or you can try running the macro described in this blog, Adobe LiveCycle Designer Cookbooks by BR001: XML Schema minOccurs="0" handling
Regards
Bruce