This is an unmodified reland of
60624b56. Nosnap bots do not block
LKGR any more: https://crbug.com/v8/9737#c10.

Original change's description:
> Disallow nullptr arguments for {CopyChars}
>
> This allows to remove special casing for the {count == 0} case, which
> was needed because {memmove} does not accept {nullptr} arguments even
> if the {count} is zero.
>
> R=leszeks@chromium.org
>
> Bug: v8:9396
> Change-Id: Iaef3cdbbffa74c2ba1c4e4501dafd943282cbcd9
> Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1807366
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63838}

TBR=leszeks@chromium.org

Bug: v8:9396
Change-Id: I6ab13575f13df060b450ff105e4b9db516671dcf
Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1809365Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63863}