fix: check routes is not empty before popping values

This commit ensures the istio-pilot charm checks that the ingress routes dictionary
it uses for reconciling VirtualServices from requirers is not empty before attempting
to pop values during a RelationBroken event.

Fixes #423

charms/istio-pilot/src/charm.py

@@ -606,7 +606,7 @@ def _get_ingress_data(self, event) -> dict:
                     f" departing application's name.  We assume that the departing application's"
                     f" is not in routes.keys='{list(routes.keys())}'."
                 )
-            else:
+            elif routes:
                 routes.pop((event.relation, event.app))
 
         return routes