[scheduler] Extra protection against empty tasks in scheduler

R=skyostil@chromium.org
BUG=789901

Change-Id: I39e845e80772571c9bac3c0d5f53fb04c3c53be5
Reviewed-on: https://chromium-review.googlesource.com/823031


Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524414}