fix: make current_billing_period nullable for Subscription notifications

Co-authored-by: Michael Woodward <michael.woodward@paddle.com>

src/Entities/Notification/NotificationSubscription.php

@@ -41,7 +41,7 @@ protected function __construct(
         public SubscriptionDiscount|null $discount,
         public CollectionMode $collectionMode,
         public BillingDetails|null $billingDetails,
-        public SubscriptionTimePeriod $currentBillingPeriod,
+        public SubscriptionTimePeriod|null $currentBillingPeriod,
         public TimePeriod $billingCycle,
         public SubscriptionScheduledChange|null $scheduledChange,
         public array $items,