HotSpot doesn't use NULL

[Kim Barrett]

A reminder that we don't use NULL in HotSpot code anymore.

We had a large batch of changes to remove uses of NULL in early 2023 (thanks
to Johan Sjölen), and another batch in early 2024 (by various folks). Since
then we've had a couple of cleanups of backsliding in this area - 
JDK-8327171
and JDK-8342860. In the last 2 weeks (since the most recent of those
cleanups), it seems we've had a flurry of new uses being introduced - 
JDK-8344917.

Hopefully we'll get a fix for JDK-8343802 soon.