Due to the previous change to that function, we can end up with
set (non-null) fields even when the overall serialized_ field is
unset. This can cause DCHECK failures (I don't think it's otherwise
observable).

Bug: chromium:1142240,v8:7790
Change-Id: I2711fae8a73438277caf7aa539f24d628b03153c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2497170
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70753}