Fixed return type declarations in RestSessionBasedFactory

Symfony 5.4 forward compatibility, for more details see ezsystems/ezplatform-rest#87

Author: ciastektk

src/bundle/DependencyInjection/Security/RestSessionBasedFactory.php

@@ -56,22 +56,22 @@ protected function createListener($container, $id, $config, $userProvider)
         return $listenerId;
     }
 
-    protected function getListenerId()
+    protected function getListenerId(): string
     {
         return 'ezpublish_rest.security.authentication.listener.session';
     }
 
-    public function getPosition()
+    public function getPosition(): string
     {
         return 'http';
     }
 
-    public function getKey()
+    public function getKey(): string
     {
         return 'ezpublish_rest_session';
     }
 
-    protected function createEntryPoint($container, $id, $config, $defaultEntryPoint)
+    protected function createEntryPoint($container, $id, $config, $defaultEntryPoint): ?string
     {
         return $defaultEntryPoint;
     }