util: fix inspect performance bug

In case an object contained a circular reference `Object.keys` was
called even though it was not necessary at all. This caused a
significant overhead for objects that contained a lot of such entries.

PR-URL: https://github.com/nodejs/node/pull/20007


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>