[Doctrine] Fix default value to null for entity manager to make fluen…

…t integration with Doctrine Registry work

src/Symfony/Bridge/Doctrine/Validator/Constraints/UniqueEntity.php

@@ -23,7 +23,7 @@
 class UniqueEntity extends Constraint
 {
     public $message = 'This value is already used.';
-    public $em = false;
+    public $em = null;
     public $fields = array();
 
     public function getRequiredOptions()