Fix #[IsCsrfTokenValid] to ensure $tokenKey is non-nullable

Author: webda2l

src/Symfony/Component/Security/Http/Attribute/IsCsrfTokenValid.php

@@ -25,7 +25,7 @@ public function __construct(
         /**
          * Sets the key of the request that contains the actual token value that should be validated.
          */
-        public ?string $tokenKey = '_token',
+        public string $tokenKey = '_token',
 
         /**
          * Sets the available http methods that can be used to validate the token.