tls: fix SNICallback without .server option

`options.server` only needs to be set when its
contents are actually being inspected.

PR-URL: https://github.com/nodejs/node/pull/17835


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>