stream: simplify Writable.write

Slightly refactors Writable.write for minor perf
and readability improvements.

PR-URL: https://github.com/nodejs/node/pull/31146


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>