.. which can return Undefined if reading out of bounds, so the return
type is ObjectRef and not StringRef (if we had torque-like union types
it'd be StringRef|OddballRef). Also change the function name to
GetCharAsStringOrUndefined.

Bug: v8:7790,chromium:1181246
Change-Id: Icf9e8fd03d11c3936e87a509b9117e547972d283
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2712965Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72952}