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>

doc/api/https.md

@@ -29,6 +29,10 @@ An [`Agent`][] object for HTTPS similar to [`http.Agent`][]. See
   Can have the same fields as for [`http.Agent(options)`][], and
   * `maxCachedSessions` {number} maximum number of TLS cached sessions.
     Use `0` to disable TLS session caching. **Default:** `100`.
+  * `servername` {string} the value of
+    [Server Name Indication extension][sni wiki] to be sent to the server. Use
+    empty string `''` to disable sending the extension.
+    **Default:** hostname or IP address of the target server.
 
     See [`Session Resumption`][] for infomation about TLS session reuse.
 
@@ -406,3 +410,4 @@ headers: max-age=0; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; p
 [`tls.createSecureContext()`]: tls.html#tls_tls_createsecurecontext_options
 [`tls.createServer()`]: tls.html#tls_tls_createserver_options_secureconnectionlistener
 [`Session Resumption`]: tls.html#tls_session_resumption
+[sni wiki]: https://en.wikipedia.org/wiki/Server_Name_Indication