test: check null proto-of-proto in util.inspect()
Add a test to check util.inspect()'s handling of a null prototype-of-an-iterable-prototype. This covers a previously uncovered code branch.
Refs: https://coverage.nodejs.org/coverage-0fd121e00c9d5987/lib/internal/util/inspect.js.html#L597
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines