As Andre Guirard pointed out in his Blog a fulltext search for an empty field is possible. The syntax to search for a none empty field is
[Feldname] is present
If the aim is to search for a empty field the search query has to be provided with a NOT. So if you want to search for a todo with no one assigned to the query would be
[Form] = "Todo" AND NOT [AssignedTo] is present
if the form is "Todo" and the person assigned to is in the field "AssignedTo".
At the marvelous Engage conference I had the honor to speak about "How to get my app on mobile?". Between the obvious choices of having a … more