add null test

Author: cjprinse-etrias

tests/DBAL/JmsJsonTypeFunctionalTest.php

@@ -89,6 +89,7 @@ public function values()
         $date = new \DateTime();
 
         return array(
+            array(null, null),
             array(1, 'integer::1'),
             array(1.25, 'double::1.25'),
             array(true, 'boolean::true'),