fix(push-notification): Add non-null assertion for notification client

- Ensure that the `client` is not null before calling `sendBulkNotifications`
- This change improves the robustness of the push notification service

Author: fulleni

lib/src/services/push_notification_service.dart

@@ -244,7 +244,7 @@ class DefaultPushNotificationService implements IPushNotificationService {
       final sendFutures = notificationsToCreate.map((notification) {
         final userDeviceTokens =
             userDeviceTokensMapForSending[notification.userId] ?? [];
-        return client
+        return client!
             .sendBulkNotifications(
               deviceTokens: userDeviceTokens,
               payload: notification.payload,