Fixes an issue with tracing empty ephemeron values of mixin types.

Bug: chromium:1056170
Change-Id: I0089df29943ba7670ec4bdfa5592a01b0ec6de04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2732025Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73185}