在下面的例子中，我希望如果typeOfThingSubject没有被释放一个值，那么现有的非空things应该从else分支返回。相反，在流构建器中，snapshot.data为null。

// in bloc constructor
final typeOfThingSubject = BehaviorSubject<TypeOfThing?>();

// at this point the above subject still wasn't emitted with data
final filteredThings = typeOfThingSubject
    .map<Iterable<Thing>>((value) {
  if (value != null) {
    return <Thing>[...];
  } else {
    // obviously this branch should be executed but that doesn't happen
    // when typeOfThingSubject wasn't emitted with an event yet
    // and truly the `typeOfThingSubject` is just declared at this point
    return things; // existing iterable
  }
});

return Bloc._( ...
  things: filteredThings, // this is the same stream `things` as in the builder below
);

// in view
StreamBuilder<Iterable<Thing>>(
  stream: bloc.things,
  builder: (context, snapshot) {
    // at this point `snapshot.data` is null
    final things = snapshot.data ?? [];
    return ListView.builder( ...

在filteredThings的定义中，它Map到BehaviorSubject<TypeOfThing?>上，在初始化的时候，主题仍然没有发送数据，那么为什么当map中的代码命名部分执行else分支不起作用呢？
我期望如果流typeOfThingSubject仍然是null，那么现有的iterable将返回，但它不以这种方式工作。