IngressPerUnitRequirer.url should return a None on relation broken

[sed-i]

Bug Description

When I removed the relation between prometheus and traefik, I noticed that web.external-url still has the ingress path.
This is because in prometheus we use self.ingress.url (IngressPerUnitRequirer.url), which in turn uses _urls_from_relation_data.

However, on relation broken, relation data can be misleading.

traefik-k8s-operator/lib/charms/traefik_k8s/v1/ingress_per_unit.py

Lines 726 to 728 in f6b150f

	self.framework.observe(
	self.charm.on[self.relation_name].relation_broken, self._handle_relation
	)

Luckily, in _handle_relation, the requirer already has up to date information:

def _handle_relation(self, event: RelationEvent):
    # ...
    self._stored.current_urls = current_urls
    # ...
    self.on.revoked.emit(self.relation, unit_name)

All we need to do™ is also look at self._stored.current_urls inside IngressPerUnitRequirer.url.

To Reproduce

1. Relate prom - trfk.
2. Remove the relation.

Environment

Juju 3.2.0. Latest charms.

Relevant log output

NTA.

Additional context

No response

[sed-i]

With recent ops, relation data should be hidden on relation broken.

[dstathis]

We certainly have a recent enough version of ops by now.