The user of the API is allowed to call data() on an empty vector.

Bug: 1026414
Change-Id: I41ea65c50b94115ace7930d75cd8f14782548d6c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1937807Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>