Vanilla relative teleport doesn't work

CMDBOOK-2152 - Reported by Carl.Fürstenberg

Trying to do the vanilla teleportation doesn't work. i.e. "/tp User ~0 ~4 ~0" returns "Number expected, string received instead"

Start by reproducing the `/tp User ~0 ~4 ~0` command and tracing the vanilla relative-teleport argument handling. No file or test is named in the issue; done means the command accepts relative coordinates without the reported “Number expected, string received instead” error.

Written by the indexing model from the issue text.