[whatwg/webidl] An undefined dictionary member in IDL->JS conversion: exist or not? (Issue #1094)

The Streams spec has [the following dictionary](https://streams.spec.whatwg.org/#dictdef-readablestreambyobreadresult):

```webidl
dictionary ReadableStreamBYOBReadResult {
  ArrayBufferView value;
  boolean done;
};
```

Here, `value` can be an ArrayBufferView or undefined per the relevant algorithms:

* https://streams.spec.whatwg.org/#byob-reader-read, "given chunk" in "close steps"
* Where the "chunk" can be undefined https://streams.spec.whatwg.org/#read-into-request-close-steps

And there's relevant WPT tests that checks the existence of `value` field even when it's `undefined`. https://github.com/web-platform-tests/wpt/blob/62c22e4362e19418022e03ca4f5bea76b998fa3f/streams/readable-byte-streams/tee.any.js#L503

And here's my question: does the IDL spec allows an `undefined` field as a result of IDL->JS conversion? And if it does, should it?

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