This is going way back for me, so I could be completely wrong, but I think that's controlled by the NilIsNull property in the XML template.
Do you have the column property "Empty String is NULL" set to ON? I believe that's what triggers the generation of the nilisnull="true" into the XML export template for that attribute. And I don't think that's dynamic - I think you need to delete and regenerate the template after you set the property.
-Paul Horan-