streams: handle null push for transform in async_iterator
when the readable side of a transform ends any for await loop on that transform stream should also complete. This fix prevents for await loop on a transform stream from hanging indefinitely.
Essential for #28501 (see https://github.com/nodejs/node/pull/28501#discussion_r299248335)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
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