For a zero-length BackingStore allocation, it is valid for the
underlying memory to be a null pointer. However, some cleanup
is still necessary, since the BackingStore may hold a reference
to the allocator itself, which needs to be released when destroying
the `BackingStore` instance.

Change-Id: I1f168079d39e4592d2fde31fbe5f705586690e85
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2169646Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67420}