util: handle null prototype on inspect

This makes sure the `null` prototype is always detected properly.

PR-URL: https://github.com/nodejs/node/pull/22331
Fixes: https://github.com/nodejs/node/issues/22141


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>