src: fix crypto.pbkdf2 callback error argument

Callbacks should always return `null` instead of `undefined` if no
error occurred.

PR-URL: https://github.com/nodejs/node/pull/18458


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>