inspect: do not show <nil> as gateway

When a network is created without gateway, i.e. --internal --disable-dns
then the gateway will be nil. But converting a nil ip to string produces
"<nil>" which is not what we want as we like an empty string there to
signal that the network has no gateway.

Fixes: #28705

Signed-off-by: Paul Holzinger <pholzing@redhat.com>