doc: document that server.address() can return null

PR-URL: https://github.com/nodejs/node/pull/32519


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>