Change kUTCTimeZoneIndex to int32_t from uint32_t
And compare index against 0.

Bug: v8:11544
Change-Id: Ib7dd55cab796d1d9a4b0e2869b098520265a04f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3632678
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80412}