stream: make async iterator .next() always resolve

See: https://github.com/nodejs/readable-stream/issues/387

PR-URL: https://github.com/nodejs/node/pull/24668


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>