Fix hook secret-changed

Author: taurus-forever

src/charm.py

@@ -861,7 +861,7 @@ def _peer_relation_changed_checks(self, event: HookEvent) -> bool:
         """Split of to reduce complexity."""
         # Prevents the cluster to be reconfigured before it's bootstrapped in the leader.
         logger.debug(f"Calling on_peer_relation_changed, event: '{event}'")
-        if event.unit is None:  # or event.unit == self.unit.name:
+        if hasattr(event, "unit") and event.unit is None:
             logger.debug(f"Early exit on_peer_relation_changed: event to itself ({event.unit})")
             return False