Lazily register prototype users

when handing out validity cells to handles; because invalidating said cells is the only time we'll need the user registrations.
Along the way, fix a corner case in WeakFixedArray, which can now be empty after the recently introduced compaction support.

Review URL: https://codereview.chromium.org/1104813004

Cr-Commit-Position: refs/heads/master@{#28047}