Make compatible with PHP 8.4

> MessageBird\Client::__construct(): Implicitly marking parameter $httpClient as nullable is deprecated, the explicit nullable type must be used instead

src/MessageBird/Client.php

@@ -147,7 +147,7 @@ class Client
      */
     protected $numbersAPIClient;
 
-    public function __construct(?string $accessKey = null, Common\HttpClient $httpClient = null, array $config = [])
+    public function __construct(?string $accessKey = null, ?Common\HttpClient $httpClient = null, array $config = [])
     {
         if ($httpClient === null) {
             $this->conversationsAPIHttpClient = new Common\HttpClient(