Hi,
It seems that  this is not supported in Lucene search engine (used in smart search module), so you would need to use some workaround. 
You can set some pre-defined value to your document type field if the field is empty (for example XXXemptyXXX) automatically. 
You can achieve this in custom tree node handler in OnBeforeUpdate or OnBeforeInsert events:
http://devnet.kentico.com/docs/devguide/treenode_handler.htmThen your smart search condition would look like this:
-documentfieldname:XXXemptyXXX
Best regards,
Miroslav Remias.