When the server's `vault` block has a default identity, we don't check the
user's Vault token (and in fact, we warn them on job submit if they've provided
one). But the validation hook still checks for a token if
`allow_unauthenticated` is set to true. This is a misconfiguration but there's
no reason for Nomad not to do the expected thing here.
Fixes: https://github.com/hashicorp/nomad/issues/19565
