Please Help! Null Date Field Can't Be Used in my Formula

Please Help! Null Date Field Can't Be Used in my Formula

I have 2 date fields:
Start Date(1st) and End Date(1st)

I want to compare the 2 fields, unless the End Date is empty. In that case I want to compare Now() (the current date) with what is in the Start Date field.

I have tried this:

If(Len('${CustomModule4.End Date(1st)}')<19,Datecomp(Now(),${CustomModule4.Start Date(1st)}),Datecomp(${CustomModule4.End Date(1st)},${CustomModule4.Start Date(1st)}))

If End Date(1st) field is empty, meaning I haven't entered any date yet - the formula doesn't work and I get no output into the formula field.

If End Date(1st) has a date entered, the formula works.

What is going on?? Is there a way to check for null date fields other than the Len method I used??