Fix an issue with rendering unit messages for diffs with no buildable

Summary: Fixes T10591. This was accidentally reverted in 148a50e48be146b88036032d1cad772d2627da28, probably when resolvign a merge/rebase.

Test Plan: Will push to production.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10591

Differential Revision: https://secure.phabricator.com/D15474