Most often, the {ValueType::Ref} constructor was called with a
constant nullability. To make things more convenient, this CL renames
{Ref} to {RefMaybeNull}, and introduces {Ref} and {RefNull}
constructors with fixed nullability.

Bug: v8:7748
Change-Id: I664ff184ca936cc752e152c3c67546d79aa24390
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732936Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81494}