[tempest-k8s] Fix error on remove-application

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

ops-sunbeam/ops_sunbeam/interfaces.py

@@ -84,7 +84,7 @@ class OperatorPeers(Object):
         )
 
     @property
-    def peers_rel(self) -> ops.model.Relation:
+    def peers_rel(self) -> ops.model.Relation | None:
         """Peer relation."""
         return self.framework.model.get_relation(self.relation_name)
 
@@ -146,11 +146,15 @@ class OperatorPeers(Object):
 
     def set_unit_data(self, settings: Dict[str, str]) -> None:
         """Publish settings on the peer unit data bag."""
+        if not self.peers_rel:
+            return
         for k, v in settings.items():
             self.peers_rel.data[self.model.unit][k] = v
 
     def all_joined_units(self) -> List[ops.model.Unit]:
         """All remote units joined to the peer relation."""
+        if not self.peers_rel:
+            return []
         return set(self.peers_rel.units)
 
     def expected_peer_units(self) -> int:
@@ -158,4 +162,6 @@ class OperatorPeers(Object):
 
         NOTE: This count includes this unit
         """
+        if not self.peers_rel:
+            return 0
         return self.peers_rel.app.planned_units()