doc: correct info in child_process.md

`child.stderr`, `child.stdin`, and `child.stdout`
are `null`, not `undefined`, if the relevant `stdio` properties
are set to anything other than 'pipe'.

PR-URL: https://github.com/nodejs/node/pull/11949


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>