Most paths filter out empty reference on cppgc::Visitor or
v8::JSVisitor level. For v8::TracedReference we may end up with empty
reference in case of ephemeron tracing which cannot perfom the null
checks on the outer visitor.

Bug: chromium:1315550, v8:12600
Change-Id: I5ebb466100a6f2cf25a75585fc2267a632497548
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3582124Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79943}