net: support passing null to listen()

This commit fixes a regression around the handling of null
as the port passed to Server#listen(). With this commit,
null, undefined, and 0 have the same behavior, which was the
case in Node 4.

Fixes: https://github.com/nodejs/node/issues/14205
PR-URL: https://github.com/nodejs/node/pull/14221


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>