Fix issue with monolog 2 type error

Author: hedii

src/GelfLoggerFactory.php

@@ -66,7 +66,7 @@ public function __invoke(array $config): Logger
 
         $handler = new GelfHandler(new Publisher($transport), $this->level($config));
 
-        $handler->setFormatter(new GelfMessageFormatter($config['system_name'] ?? null, null, null));
+        $handler->setFormatter(new GelfMessageFormatter($config['system_name'] ?? null, null, ''));
 
         foreach ($this->parseProcessors($config) as $processor) {
             $handler->pushProcessor(new $processor);