Supervisior Hierarchy with No value at the some approval level

We are having customer, need your insight to fulfill his requirement -

Customer using the approved UI when creating approval rules.

He is building a Requisition approval rules based on a Supervisor Hierarchy, and He need to use the "Requester Level 4 Supersior" field in order to match the client's logic.
The issue is that, not all of the future requesters have a Level 4 Supervisor, so the the entire approval structure fails when ever a requester without a level 4 supervisor is trying to submit a requisition, since the system cannot compare "IS IN" or "IS NOT IN" values to NULL.

So the question is: is it possible to use SQL functions (like NVL) within the approval rules in order to prevent an error?
For example: NVL("Requester Level 4 Supersion",1) IS IN ('John', 'Tom', 'Jason').

Could you please help us to capture this requirement.

Answere -
In order to avoid the error, can they add a 'is not blank' condition in the approval rule and re-try the same use case ?

The "is not blank" condition must come before the "equals" condition.

e.g Requisition Amount Equals 80 and Requester Level 4 Supervisor Is not blank and Requester Level 4 Supervisor Equals LastName,FirstName