dns: cleanup validation

The `validateString()` validator should cleanup the validation and
keep consistency.

PR-URL: https://github.com/nodejs/node/pull/40061


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>