nullptr objects are considered alive to allow weakness to be used from
stack while running into a conservative GC. Treating nullptr as dead
would mean that e.g. custom collectins could not be strongified on
stack.

Bug: chromium:1231286
Change-Id: Ibeddef18fcbae366c3f54304bf36b75c47bd74ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3085280
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76229}