Invalid stun/turn servers when giving empty public address in Anbox Cloud Appliance init

[Michele Lo Russo]

After installing the appliance, I received this error in the console of the browser when joining a session:

Uncaught DOMException: Failed to construct 'RTCPeerConnection': ICE server parsing failed: Invalid hostname format
    at AnboxWebRTCManager._onWsOpen (http://localhost:8406/js/sdk/anbox-stream-sdk.js:2716:16)

Indeed, by inspecting the list of ICE servers received, it turns out that an empty address was being sent:

[
    {
        "urls": [
            "stun:stun.l.google.com:19302"
        ]
    },
    {
        "urls": [
            "stun::5349",
            "turn::5349"
        ],
        "username": <omitted>,
        "credential": <omitted>
    }
]

Steps to reproduce:

1. Do a fresh init of the anbox cloud appliance
2. Provide an empty public IP address
3. Check that the /var/snap/anbox-cloud-appliance/common/agent/config.yaml file ends up with this content:

coturn:
  urls:
  - stun::5349
  - turn::5349