Trying to write an empty byte array currently invokes undefined behavior
if the byte array is null. That's the case when trying to write an empty
vector for example. Copying zero bytes from nullptr is defined as UB for
memcpy.

Change-Id: I6f7e920c1e19e8b2e3779bbc1c0ad79fc8bd6e98
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2148789
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67168}