Refund returns null on success

Refund request, if successful, returns 200 OK with empty body: https://developer.ipay.ge/v1/#tag/Refund
Return type for refund() should be nullable.

src/Contracts/IPay.php

@@ -229,7 +229,7 @@ public function token()
      * @return \stdClass
      * @throws \GuzzleHttp\Exception\GuzzleException
      */
-    public function refund(string $order_id, int $amount, string $token = null): \stdClass
+    public function refund(string $order_id, int $amount, string $token = null): ?\stdClass
     {
         $url = config('ipay.url') . '/checkout/refund';