luci-base: properly handle undefined IPv6 local-address information

luci-base: properly handle undefined IPv6 local-address information

If IPv6 prefix assignment is disabled, the "local-address" structure
might exist, but be empty which causes the adress formatting in the
network model class to bail out.

Verify the completeness of the "local-address" structure before using
it in order to avoid runtime errors.

Fixes #1657.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>