Similar to https://crrev.com/c/v8/v8/+/2270548, we can add an early
return since we will not be creating thin strings while
single_generation is on.

Bug: v8:7790, v8:10614
Change-Id: Ib6ccc00bc79058daa163920a944ad328515f667e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2948888
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75042}