R1:298a30e647a8

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

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