While uncommon, I believe it is possible to have an empty JSON Pointer (i.e. ""), referencing the root of the document (see examples in the RFC).

Here is a regression test:

resource = Resource.opaque(contents={"foo": "baz"})
resolver = Registry().resolver()
assert resource.pointer("", resolver=resolver).contents == {"foo": "baz"}

Current behavior:

referencing.exceptions.PointerToNowhere: '' does not exist within {'foo': 'baz'}