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: nodejs/node#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>

doc/api/child_process.md

@@ -1067,7 +1067,7 @@ added: v0.1.90
 A `Readable Stream` that represents the child process's `stderr`.
 
 If the child was spawned with `stdio[2]` set to anything other than `'pipe'`,
-then this will be `undefined`.
+then this will be `null`.
 
 `child.stderr` is an alias for `child.stdio[2]`. Both properties will refer to
 the same value.
@@ -1085,7 +1085,7 @@ A `Writable Stream` that represents the child process's `stdin`.
 continue until this stream has been closed via `end()`.*
 
 If the child was spawned with `stdio[0]` set to anything other than `'pipe'`,
-then this will be `undefined`.
+then this will be `null`.
 
 `child.stdin` is an alias for `child.stdio[0]`. Both properties will refer to
 the same value.
@@ -1140,7 +1140,7 @@ added: v0.1.90
 A `Readable Stream` that represents the child process's `stdout`.
 
 If the child was spawned with `stdio[1]` set to anything other than `'pipe'`,
-then this will be `undefined`.
+then this will be `null`.
 
 `child.stdout` is an alias for `child.stdio[1]`. Both properties will refer
 to the same value.