Handle nil output from hooks

There was a bug where Overcommit would crash if a hook returned no
output but failed/warned in some way. Fix it by checking for nil.

Change-Id: I251d2cdbbdd71ffea536e2aea7e2cfeff04e0dac
Reviewed-on: http://gerrit.causes.com/35699
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>