Charm should block if external hostname isn't set in subdomain routing mode

[sed-i]

Bug Description

When traefik is running in subdomain routing mode and without an external hostname set, invalid ingress URLs are generated, such as:
test-prom-0.10.43.8.206

To Reproduce

Deploy this bundle and then juju run trfk/0 show-proxied-endpoints.

bundle: kubernetes
applications:
  ca:
    charm: self-signed-certificates
    channel: edge
    revision: 37
    scale: 1
    constraints: arch=amd64
  external-ca:
    charm: self-signed-certificates
    channel: edge
    revision: 37
    scale: 1
    constraints: arch=amd64
  prom:
    charm: prometheus-k8s
    channel: edge
    revision: 150
    series: focal
    resources:
      prometheus-image: 128
    scale: 1
    constraints: arch=amd64
    storage:
      database: kubernetes,1,1024M
    trust: true
  trfk:
    charm: traefik-k8s
    channel: edge
    revision: 157
    series: focal
    resources:
      traefik-image: 148
    scale: 1
    options:
      routing_mode: subdomain
    constraints: arch=amd64
    storage:
      configurations: kubernetes,1,1024M
relations:
- - prom:ingress
  - trfk:ingress-per-unit
- - external-ca:certificates
  - trfk:certificates

Environment

Model  Controller  Cloud/Region        Version  SLA          Timestamp
test   k8s         microk8s/localhost  3.1.5    unsupported  14:31:33-04:00

App          Version  Status  Scale  Charm                     Channel  Rev  Address        Exposed  Message
ca                    active      1  self-signed-certificates  edge      37  10.152.183.57  no       
external-ca           active      1  self-signed-certificates  edge      37  10.152.183.66  no       
prom         2.46.0   active      1  prometheus-k8s            edge     150  10.152.183.73  no       
trfk         2.10.4   active      1  traefik-k8s               edge     157  10.43.8.206    no       

Unit            Workload  Agent  Address       Ports  Message
ca/0*           active    idle   10.1.166.104         
external-ca/0*  active    idle   10.1.166.126         
prom/0*         active    idle   10.1.166.83          
trfk/0*         active    idle   10.1.166.99          

Relation provider         Requirer               Interface         Type     Message
external-ca:certificates  trfk:certificates      tls-certificates  regular  
prom:prometheus-peers     prom:prometheus-peers  prometheus_peers  peer     
trfk:ingress-per-unit     prom:ingress           ingress_per_unit  regular  
trfk:peers                trfk:peers             traefik_peers     peer     

Relevant log output

$ juju run trfk/0 show-proxied-endpoints
Running operation 8 with 1 task
  - task 9 on unit-trfk-0

Waiting for task 9...
proxied-endpoints: '{"prom/0": {"url": "https://test-prom-0.10.43.8.206/"}}'

Additional context

No response