Just as an FYI, if your looking to check for an active null, you can check the field type. Looking at the xlateInType value for the field in question will tell you if it is an active null.
Active Null looks like this….
XlateInTypes:>null<
XlateInVals:>{}< (|""|)
Passive Null looks like this…
XlateInTypes:>ch<
XlateInVals:>{}< (||)