Fix "Reviewers" validation issue with empty reviewers

Summary: Fixes T11021.

Test Plan: Created a revision without reviewers.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11021

Differential Revision: https://secure.phabricator.com/D15968

src/applications/differential/customfield/DifferentialReviewersField.php

@@ -237,6 +237,10 @@ final class DifferentialReviewersField
   }
 
   public function validateCommitMessageValue($value) {
+    if (!$value) {
+      return;
+    }
+
     $author_phid = $this->getObject()->getAuthorPHID();
 
     $config_self_accept_key = 'differential.allow-self-accept';