Always show build target tabs, even if they have no data

Summary:
Ref T8096. Hit this on IRC:

epriestley: what's in the Messages tab?
jdoe: what Messages tab??!
epriestley: ughhhh

Test Plan: Clicked "Messages" tab, saw helpful "no messages" message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8096

Differential Revision: https://secure.phabricator.com/D13846