child_process: ignore undef/proto values of env

At present, undefined values of env option will be transferred as
a "undefined" string value, and values in prototype will also be
included, which are not usual behaviors.

This commit prevents those to be transferred to the environment of
the child process.

PR-URL: https://github.com/nodejs/node/pull/15089
Fixes: https://github.com/nodejs/node/issues/15087


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>