Fix undefined variable warning in unit test field

Summary:
See D1295. $unit_messages may be undefined.

I'll see if I can improve the visibility of warnings, the red dot in DarkConsole
is easy to miss right now. See T734.

Test Plan: Loaded a revision with no unit failures, didn't receive a warning.

Reviewers: nh, btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1306

src/applications/differential/field/specification/unit/DifferentialUnitFieldSpecification.php

@@ -67,7 +67,6 @@ final class DifferentialUnitFieldSpecification
           $postponed_count++;
         }
       }
-    }
       if ($unit_messages) {
         $utail =
           '<div class="differential-unit-block">'.
@@ -76,6 +75,7 @@ final class DifferentialUnitFieldSpecification
             '</ul>'.
           '</div>';
       }
+    }
 
     if ($postponed_count > 0 &&
         $diff->getUnitStatus() == DifferentialUnitStatus::UNIT_POSTPONED) {