Fix a possible count(null) in DifferentialRevisionActionTransaction

Summary:
This change prevents the following error when using PHP 7.2:

```
ERROR 2: count(): Parameter must be an array or an object that implements Countable at [/usr/local/lib/php/phabricator/src/applications/differential/xaction/DifferentialRevisionActionTransaction.php:132]
```

A similar issue was fixed in D18964

Test Plan: Tested in a live system.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19242

src/applications/differential/xaction/DifferentialRevisionActionTransaction.php

@@ -80,7 +80,7 @@ abstract class DifferentialRevisionActionTransaction
     DifferentialRevision $revision) {
     return array(
       array(),
-      null,
+      array(),
     );
   }