Empty servername no longer defaults to public-address

Since r46 of the apache2 charm, the servername config key no longer defaults to public-address. It remains empty and that breaks deployments:

options:
  servername:
    type: string
    default: ''
    description: ServerName for vhost, defaults to the units public-address

Before r46 config_get had an exception for servername:
hooks/hooks.py:78:
def config_get(scope=None):
(...)
        if not config_data["servername"]:
            config_data["servername"] = run(["unit-get", "public-address"]).rstrip()

In r46, the charm started using charm helpers and that config_get obviously has no such exception.

On a side note, the commit for r46 didn't mention this huge change of using charm helpers ;)