rendering: Fix running the metric on non-renderer stories.
rendering: Fix running the metric on non-renderer stories.

It is possible that some tests will not have any renderers, and
so wouldn't have any raster threads either. Handle such cases
gracefully.

BUG=chromium:945709

Change-Id: Ib03214a199b28b6d6cbe52d6f3141cd00b3fa988
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/1554041
Reviewed-by: Caleb Rouleau <58bdb6276c01e3ed53e660bf706aa902204feb0d@chromium.org>
Commit-Queue: Sadrul Chowdhury <be235e35a6c144329e4aa1b0fbe2c3f8c71f3676@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult