As far as I can tell, SQL data sources do not distinguish between NULL and blank values. Both are simply blank in the XML data.
The QueryDB web service that's part of DBXL does allow drawing this distinction: null values are returned with an xsi:nil attribute in the XML, while blank values are simply blank, without an xsi:nil attribute. Switching to QueryDB would probably be overkill for just this one feature, but on the other hand, QueryDB does offer several benefits compared to SQL data connections.