The trait is expected to return a nullptr for the base address. This
is required for ephemeron tracing to trigger eagerly tracing a value.

This will be required when Blink uses a type alias to TracedReference.

Bug: v8:12165
Change-Id: Ibe142eaff41616c9de6ae0db9878f8489a5e4142
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3253345Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77619}