Fix the doctrine entity autocompleter field. (Revert the changes from…

… PR genemu#68, which brought the regression).

Form/Doctrine/Type/AjaxEntityType.php

@@ -49,7 +49,7 @@ public function getDefaultOptions(array $options)
             'class'         => null,
             'property'      => null,
             'query_builder' => null,
-            'choices'       => array(),
+            'choices'       => null,
             'ajax'          => false
         );