Always return a date from RequestModel::getLastApprovalTime(). Even when null.

application/modules/requests/models/RequestModel.php

@@ -933,7 +933,7 @@ class Requests_RequestModel extends Unl_Model
     	if ($time > 0) {
     		return new Zend_Date($time);
     	}
-    	return null;
+    	return new Zend_Date(0);
     }
     
     public function getLastApprovalTimestamp()