cert-provider relation departed results in hook-failure

if the `charm.docker-registry.client-configured` flag is set when this relation departs, the charm fails with this hook-failure:

unit-docker-registry-0: 16:11:59 ERROR unit.docker-registry/0.juju-log cert-provider:17: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-docker-registry-0/charm/reactive/docker_registry.py", line 232, in remove_certs
    configure_client()
  File "/var/lib/juju/agents/unit-docker-registry-0/charm/reactive/docker_registry.py", line 138, in configure_client
    registry.set_registry_config(netloc, **data)
AttributeError: 'NoneType' object has no attribute 'set_registry_config'

unit-docker-registry-0: 16:11:59 ERROR juju.worker.uniter.operation hook "cert-provider-relation-departed" (via explicit, bespoke hook script) failed: exit status 1