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