... in favour of an optional return type for MapRef::prototype().
This also eliminates one kind of use of ShouldHaveBeenSerialized(),
which I want to get rid of entirely.

Bug: v8:7790
Change-Id: I031f067d644570e5c8aaeaf94c5ff69ff0515a99
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2874456
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74385}