Specify what happens when relative json pointer is `0`

[Relequestual]

Unless I'm missing something, we don't actualy specify what happens when a Relative JSON Pointer is 0 in the Relative JSON Pointer specification.

We hint and show examples, but it's not strictly specified.

We say...

If this value is more than zero, then the following steps are repeated that number of times...

But we do not say what happens when the value is zero.

This has resulted in a question being raised on our slack.