There's a dozen "contains" "greater than" "less than" etc options in the dropdown, "empty" or "null" should be one of them.
Why not do something along the lines of "if (X) is equal to (null)?
When I tried to put "null" into the expressions box, it said "invalid expression" even when it popped up in the picker and I clicked it from there. I was able to find an expression to paste in from Google from someone else with a similar problem that worked, but I had to try multiple "solutions" to find something it would accept. It would be nice to have something that just worked from the dropdown.
... And also it would be nice to have choices "is true" and "is false"