Change prefered function to NVL from NULLIF to IFFNUL

Why is this change being made?

In the sql style guide it's referenced that it's prefered to use NULLIF to NVL. I think it should be IFNULL since is the synonym to NVL

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what