No need to return an empty map. Return a JSObject instead.

Bug: v8:5933
Change-Id: I9fb727c5e1920ba94fd3d5e7ef2a7d9d602f56d8
Reviewed-on: https://chromium-review.googlesource.com/506194Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45350}