Fix PHP 8.3 deprecation warning in ApiException

Optional parameter $message declared before required parameter $response is implicitly treated as a required parameter in /home/runner/work/gw2api/gw2api/src/Exception/ApiException.php on line 12

src/Exception/ApiException.php

@@ -9,7 +9,7 @@ class ApiException extends Exception {
     /** @var ResponseInterface $response */
     protected $response;
 
-    public function __construct( $message = "", ResponseInterface $response ) {
+    public function __construct( $message, ResponseInterface $response ) {
         $this->response = $response;
 
         parent::__construct( $message, $response->getStatusCode() );