stream: fix readable behavior for highWaterMark === 0

Avoid trying to emit 'readable' due to the fact that
state.length is always >= state.highWaterMark if highWaterMark is 0.
Therefore upon .read(0) call (through .on('readable')) stream assumed
that it has enough data to emit 'readable' even though
state.length === 0 instead of issuing _read(). Which led to the TTY
not recognizing that someone is waiting for the input.

Fixes: https://github.com/nodejs/node/issues/20503
Refs: https://github.com/nodejs/node/pull/18372

PR-URL: https://github.com/nodejs/node/pull/21690


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>