Will allow running "null" Mock ICD tests.

Also fixes a crash running the Mock ICD with the overlay.

Bug: angleproject:5736
Change-Id: I42e3943a0de00dbab3ee125f1946afd22b38fec6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2809096Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>