Condition builder with null value

Ok, I've been at this for days…

When making an audit rule, I have 4 fields. 2 are predicatable text(select 1 of three values), one is a username, and one is a date.
I'm trying to make a rule to determine if any of the fields are empty as they all need to be filled in otherwise the policy should violate.

I can make a simple "Not Equals" for the 2 predictable text fields BUT the username and date fields I can't. So far I've tried

equals null

does not equal null

(ironically it returns both as false)

equals $(null)

contains null

contains NULL