child_process: refactor internal/child_process.js

* Prefer === to == where possible
* Remove condition that will always be false
* Prefer for-loop statements to forEach where possible for perfomance reasons

PR-URL: https://github.com/nodejs/node/pull/11366


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>