When running mksnapshot standalone, no callback is registered to
serialize embedder fields. In this case they are most probably empty as
there is no embedder to set them, so provide a default serialization
that keeps them as nullptr.

Bug: v8:8822
Change-Id: I9219f2755488aa1473959c3ee75e9d4f47202359
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1507677
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60094}