R1:73b68bc2a619

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…