[v2.1] Need to guard against a relation broken event

[sed-i]

When removing an app that was related to self-signed-certificates, the following error is raised:

  File "/var/lib/juju/agents/unit-am-0/charm/lib/charms/tls_certificates_interface/v2/tls_certificates.py", line 1366, in _on_relation_broken
    if not relation.data[relation.app].get("certificates"):  # type: ignore[index]
  File "/var/lib/juju/agents/unit-am-0/charm/venv/ops/model.py", line 1346, in __getitem__
    raise KeyError(
KeyError: 'Cannot index relation data with "None". Are you trying to access remote app data during a relation-broken event? This is not allowed.'

Seems like there's a missing guard:

if not relation.app or not relation.app.name:
    # Handle edge case where remote app name can be missing, e.g.,
    # relation_broken events.

Related: canonical/operator#888