[4.2] Remove useless negated is_null

The origin of this PR comes from 7dad0db1bded77ef1a4ec0769355aadea61473c2.
In many cases, a negated `! is_null` check is done, as a single `is_null` check could be sufficient and more performant.
I've rolled back the formatting @taylorotwell and also squashed my commits