The next built-in function throws a "StopIteration" when iterator is exhausted
which will crash and stop the test runner. Asking it to return a "None" instead will
fix the issue.

Change-Id: I3ca4ff8f28f359798c9faf6f1ffbbaa6bb95f156
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524719
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60294}