Ephemerons are based around WeakMember which may just be null at the
time the pair is considered for liveness. Bail out of marking for null
keys, as they write barrier would anyways make the value strong when
marking the key.

Bug: chromium:1056170
Change-Id: If8775a370824b88fc67fa479a0c0893985fbf5f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2692571Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72714}