Fix an issue with attempting to index comments on packages

Summary: See rPcd14194a329788d5fff6365bcade278fd18f3612 for a similar change. Implement `getApplicationTransactionCommentObject()` to return `null` explicitly.

Test Plan: Ran `bin/search index --type ownerspackage`, got indexing after change.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18715

src/applications/owners/storage/PhabricatorOwnersPackageTransaction.php

@@ -15,4 +15,8 @@ final class PhabricatorOwnersPackageTransaction
     return 'PhabricatorOwnersPackageTransactionType';
   }
 
+  public function getApplicationTransactionCommentObject() {
+    return null;
+  }
+
 }