fix: payment_method_id can be string or null (#77)

The API docs state that the payment_method_id can be a `string` or `null` - but the PHP code only expects a string.

The value is `null` when the payment amount has been reduced to zero by a discount.

src/Entities/Shared/TransactionPaymentAttempt.php

@@ -20,7 +20,7 @@ class TransactionPaymentAttempt
      */
     private function __construct(
         public string $paymentAttemptId,
-        public string $paymentMethodId,
+        public string|null $paymentMethodId,
         public string $storedPaymentMethodId,
         public string $amount,
         public PaymentAttemptStatus $status,