The script was crashing because of a missing 'email' value from some
gerrit results. Harden against the missing value.

BUG=None

Change-Id: I0f267f130e60e56d532ac2ec1a99ab108a3181e9
Reviewed-on: https://chromium-review.googlesource.com/1161504Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>