util: fix iterable types with special prototype

The fallback should only be taken for a null prototype. If an
iterable data type (e.g., Array) has a prototype without
`Symbol.iterator`, just try the best to visualize it as object.

PR-URL: https://github.com/nodejs/node/pull/25457
Fixes: https://github.com/nodejs/node/issues/25451


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>