luci-app-firewall: fix zone network default

When a `config zone` section lacks an `option network` or `list network`
setting, its contained interface list defaults to the name of the zone,
e.g. a zone named `foo` will implicitely contain the network `foo` unless
a deviating or empty `option network` is specified.

Adjust the zones.js model accordingly to reflect that implicit default.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>