Previously on application remove,
tempest-k8s charm would error at the relation-broken hook,
causing the removal to fail:
```
tempest/0* error idle 10.1.169.40 hook failed: "identity-ops-relation-broken"
```
with traceback:
```
File "/var/lib/juju/agents/unit-tempest-0/charm/src/handlers.py", line 609, in _on_provider_goneaway
self.charm.set_tempest_ready(False)
File "/var/lib/juju/agents/unit-tempest-0/charm/./src/charm.py", line 283, in set_tempest_ready
self.peers.set_unit_data({TEMPEST_READY_KEY: "true" if ready else ""})
File "/var/lib/juju/agents/unit-tempest-0/charm/lib/ops_sunbeam/relation_handlers.py", line 625, in set_unit_data
self.interface.set_unit_data(settings)
File "/var/lib/juju/agents/unit-tempest-0/charm/lib/ops_sunbeam/interfaces.py", line 150, in set_unit_data
self.peers_rel.data[self.model.unit][k] = v
AttributeError: 'NoneType' object has no attribute 'data'
```
This fixes the issue for tempest (and potentially if similar issues
are present on other sunbeam charms),
by checking if peels_rel exists because trying to interact with it.
Change-Id: I5e01c1dba719a63160f0452282d57b9eae14b41b
539f36817f