Ceph Dashboard fails if vault is using a self-signed root CA

If vault is configured with auto-generate-root-ca-cert=True then a chain is not passed to the dashboard which results in:

Traceback (most recent call last):
  File "./src/charm.py", line 597, in <module>
    main(CephDashboardCharm)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/main.py", line 406, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/main.py", line 140, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 278, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 722, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 767, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/interface_tls_certificates/ca_client.py", line 687, in _on_relation_changed
    self.ready_events[request_type].emit()
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 278, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 722, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/ops/framework.py", line 767, in _reemit
    custom_handler(event)
  File "./src/charm.py", line 427, in _configure_dashboard
    self._configure_tls()
  File "./src/charm.py", line 521, in _configure_tls
    key, cert, ca_cert = self._get_tls_from_relation()
  File "./src/charm.py", line 492, in _get_tls_from_relation
    self.ca_client.root_ca_chain.public_bytes(
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/interface_tls_certificates/ca_client.py", line 340, in root_ca_chain
    return self._get_certificate(self._stored.root_ca_chain)
  File "/var/lib/juju/agents/unit-ceph-dashboard-0/charm/venv/interface_tls_certificates/ca_client.py", line 315, in _get_certificate
    raise CAClientError(WaitingStatus,
interface_tls_certificates.ca_client.CAClientError