Returns the parent container IRI of iri, or null if iri is the storage root (no parent).
null
Examples:
https://pod.example/foo/bar/baz → https://pod.example/foo/bar/
https://pod.example/foo/bar/baz
https://pod.example/foo/bar/
https://pod.example/foo/bar/ → https://pod.example/foo/
https://pod.example/foo/
https://pod.example/ → null
https://pod.example/