Follow-up that continues to make StringRefs's methods return optional
values.

Bug: v8:7790
Change-Id: I34f907810747216b047a3da8db035cf4f62728c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615162Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71982}