tls: permit null as a cipher value

Allow null along with undefined for cipher value.

Fixes: https://github.com/nodejs/node/issues/36292

PR-URL: https://github.com/nodejs/node/pull/36318


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>