src: accept single argument in getProxyDetails

This makes sure this function stays backwards compatible in case
it's accessed through the binding directly.

Refs: https://github.com/nodejs/node/pull/29947#issuecomment-562987888

PR-URL: https://github.com/nodejs/node/pull/30858
Refs: https://github.com/nodejs/node/pull/30767


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>