Hide build target messages if there are no messages for the target
Summary: Ref T1049. This hides the build target messages area if there are no messages for the target. Since most of the time a build target won't recieve any messages, this area is confusing because it's always empty.
Test Plan: Viewed a build, saw the empty build target message areas disappear.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T1049
Differential Revision: https://secure.phabricator.com/D10092