It is not possible to return any blank fields with filters, unless you are given the option in the operator dropdown.  The operator drop down list contains, for example, equals or does not equal.

In Form Submitted filters there is the option to search for the result of an empty field like this:

 

For more information on filters, search this knowledge base for the search term "Filters".  Articles have been written for a variety of CRMs with which we integrate.