Merge pull request #535 from ably/ECO-4885/fix-push-serialization

fix: serialization for push notification

lib/src/push_notifications/src/notification.dart

@@ -26,6 +26,6 @@ class Notification {
   /// Creates an instance from the map.
   factory Notification.fromMap(Map<String, dynamic> map) => Notification(
         body: map[TxNotification.body] as String?,
-        title: map[TxNotification.title] as String,
+        title: map[TxNotification.title] as String? ?? '',
       );
 }