Handle the serialization in case the buffer is not yet set up.

Bug: angleproject:6093
Change-Id: I06a8e15d181c4ec514a334f952ce29a4026970a6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2982549
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>