Generally when I find for an empty field I just use an equal sign. We were working on a find routine, which I did for another system where I put a few global fields at the top of a list view. Then in the script set a variable a calculation like this.
Case ( not IsEmpty ( WD.TS::zG_DistrictID ) ; WD.TS::zG_DistrictID ; "*" )
It worked find in that system because all of the fields will always have a value. Not so in the other system.
However, In find mode operator dropdown shows this...
I read zero to mean empty but it doesn't work that way. I think it should say one or more characters.