Fix changeset construction special case for empty commits in pre-commit hooks

Summary: Fixes T13155. Ref T13151. A recent change (D19455) changed the return format here, but I missed this special case for empty commits.

Test Plan:
  - T13155 has a good set of reproduction instructions.
  - Pushed an empty commit.
    - Before: bunch of warning log spew.
    - After: clean logs.

Reviewers: amckinley, avivey

Reviewed By: avivey

Maniphest Tasks: T13155, T13151

Differential Revision: https://secure.phabricator.com/D19500

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -1212,7 +1212,7 @@ final class DiffusionCommitHookEngine extends Phobject {
 
     if (!strlen($raw_diff)) {
       // If the commit is actually empty, just return no changesets.
-      return array();
+      return array(array(), 0);
     }
 
     $parser = new ArcanistDiffParser();