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