Fix type error in Registry when no entity managers are configured

Author: Bosswell

config/dbal.php

@@ -36,7 +36,7 @@
         ->set('doctrine.dbal.events.oracle_session_init.class', OracleSessionInit::class)
         ->set('doctrine.class', Registry::class)
         ->set('doctrine.entity_managers', [])
-        ->set('doctrine.default_entity_manager', null);
+        ->set('doctrine.default_entity_manager', '');
 
     $container->services()