[web snapshot] Support empty objects

This CL fixes the deserialization of the map for empty objects, so that
the initial empty map is used.

Bug: chromium:1213851, v8:11525, v8:11706
Change-Id: I37de0b147b9c89ead9c96f776e5fbf88da4630cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2928192


Commit-Queue: Vicky Kontoura <vkont@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74880}