[autotest] Bugfix to string formatting of num=None

Handle num=None as special case of string formatting
in task.py

BUG=chromium-os:38179
TEST=run suite_scheduler.py --sanity, see no string formatting
exception

Change-Id: Ibe96a4b09cb218875a68a689a327f35b5061c443
Reviewed-on: https://gerrit.chromium.org/gerrit/41933
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
1 file changed