Make `Rx<T>().stream` null-safe

@@ -109,7 +109,7 @@ mixin RxObjectMixin<T> on NotifyManager<T> {
     return _value;
   }
 
-  Stream<T?> get stream => subject.stream;
+  Stream<T> get stream => subject.stream;
 
   /// Binds an existing `Stream<T>` to this Rx<T> to keep the values in sync.
   /// You can bind multiple sources to update the value.