Document getPartialReference() properly

According to the current implementation that method also returns `null`,
however the interface's documentation was incorrect.

Ref: https://github.com/doctrine/doctrine2/blob/v2.6.2/lib/Doctrine/ORM/EntityManager.php#L514-L516

lib/Doctrine/ORM/EntityManagerInterface.php

@@ -174,7 +174,7 @@ interface EntityManagerInterface extends ObjectManager
      * @param string $entityName The name of the entity type.
      * @param mixed  $identifier The entity identifier.
      *
-     * @return object The (partial) entity reference.
+     * @return object|null The (partial) entity reference.
      */
     public function getPartialReference($entityName, $identifier);