Hide empty build logs

Summary: This automatically hides any empty build logs from Harbormaster, so that they do not appear.

Test Plan: Viewed a build plan where the logs were empty and didn't see them appear.

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10091