http: `servername === false` should disable SNI

There is no way to disable SNI extension when sending a request to HTTPS
server. Setting `options.servername` to a falsy value would make Node.js
core override it with either hostname or ip address.

This change introduces a way to disable SNI completely if this is
required for user's application. Setting `options.servername` to ``
in `https.request` would disable overrides and thus disable the
extension.

PR-URL: https://github.com/nodejs/node/pull/27316


Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>