Fixes an incorrect assert that is hit when trying to find a program
output when none are present. It is valid to not have an output, so we
should not check for an output when there are none.

Bug:angleproject:2283
Change-Id: Ia640482870c6ee589a933b989272177760237e3b
Reviewed-on: https://chromium-review.googlesource.com/825957
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>