I have the same issue with single selection fields. I have to check that none of the potentially many possible field are selected.
If I have a single selection field called resolve with possible values of (nil, resolved, unresolved) I would think that I could do something like:
!resolved:resolved to catch both unresolved and nil values.
However, the only way that I can find that works is checking for “!resolved:resolved AND !resolved:unresolved”