notif test: Allow scheduling mock notification tap events before subscription

Initialize StreamController early, this will allow scheduling mock
notification tap events (via `addNotificationTapEvent`) even before
`notificationTapEventsStream` is called.

Author: rajveermalviya

test/model/binding.dart

@@ -779,16 +779,16 @@ class FakeNotificationPigeonApi implements NotificationPigeonApi {
   Future<NotificationDataFromLaunch?> getNotificationDataFromLaunch() async =>
     _notificationDataFromLaunch;
 
-  StreamController<NotificationTapEvent>? _notificationTapEventsStreamController;
+  late final _notificationTapEventsStreamController =
+    StreamController<NotificationTapEvent>();
 
   void addNotificationTapEvent(NotificationTapEvent event) {
-    _notificationTapEventsStreamController!.add(event);
+    _notificationTapEventsStreamController.add(event);
   }
 
   @override
   Stream<NotificationTapEvent> notificationTapEventsStream() {
-    _notificationTapEventsStreamController ??= StreamController();
-    return _notificationTapEventsStreamController!.stream;
+    return _notificationTapEventsStreamController.stream;
   }
 }