*Scan: Fix trace events reporting

If mutator enters the safepoint too late when scanning has already
finished, it will bail out immediately. However, the trace scopes that
were not entered will nevertheless be mistakenly reported (as nulls).
This CL fixes it.

Bug: 11297512
Change-Id: Ib53df5be6398b20409e4a9f39beb4b8ee33ed6b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3139814


Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918504}