Re: [whatwg/streams] Reconnecting a Stream after creating it - possible? (#1004)

Very interesting.

Since both _preventAbort_ and _preventCancel_ are `true`, the list of actions in `ReadableStreamPipeTo`'s _abortAlgorithm_ is empty. The action passed to "shutdown with an action" consists of "waiting for all of the actions in _actions_". This is defined [here](https://www.w3.org/2001/tag/doc/promises-guide/#waiting-for-all).

However, the steps in that algorithm do not account for the case where _promises_ is an empty list! It attaches a _fulfillmentHandler_ to every promise in _promises_, and checks whether we're done **only inside that handler**. But if no promises were passed in, then no handlers are attached and we never check whether we're done.

The fix is obvious: if the list of promises passed to the "wait for all" algorithm is empty, then it should immediately call _successSteps_ with an empty list. I'll make a PR for [the reference implementation](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/helpers.js#L166) and [the polyfill](https://github.com/MattiasBuelens/web-streams-polyfill/blob/6e8a378c807e155b6921f0f498a49de5cdf2ed5b/src/lib/helpers.ts#L173), and I'll try to write a spec change for [the promises guide](https://github.com/w3ctag/promises-guide).

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