Improve type hint for start method in RefreshTokenAuthenticator

- Update the type hint of the  parameter in the start method from AuthenticationException to ?AuthenticationException.
- Enhance code clarity and ensure consistency with nullable parameter type definition.

Security/Http/Authenticator/RefreshTokenAuthenticator.php

@@ -178,7 +178,7 @@ public function onAuthenticationFailure(Request $request, AuthenticationExceptio
         return $this->failureHandler->onAuthenticationFailure($request, $exception);
     }
 
-    public function start(Request $request, AuthenticationException $authException = null): Response
+    public function start(Request $request, ?AuthenticationException $authException = null): Response
     {
         $event = new RefreshTokenNotFoundEvent(
             new MissingTokenException('JWT Refresh Token not found', 0, $authException),