Metal: Handle empty vertex buffers Use the current attribute value when the bound array buffer has no data to avoid Metal Validation errors. Drive-by: do not emit empty client data upload commands. Fixed: angleproject:40050025 Change-Id: I813725322c52bc16ca432f617a40fe7cee24dd6d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5569091 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>