[FrameworkBundle] fix merge

Author: nicolas-grekas

Resources/config/serializer.php

@@ -176,6 +176,7 @@
             ->tag('serializer.encoder')
 
         ->set('serializer.encoder.json', JsonEncoder::class)
+            ->args([null, null])
             ->tag('serializer.encoder')
 
         ->set('serializer.encoder.yaml', YamlEncoder::class)