Since we currently only fill the constants table if we're serializing,
we should only create & finalize the table in that case. Otherwise,
leave it initialized to empty_fixed_array.

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:6666
Change-Id: I6ecbfac9dc9a9dac7ff0f11331be09b1cbfb4c18
Reviewed-on: https://chromium-review.googlesource.com/948490Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51727}