DNSPolicy: defaultGeo should be required in the DNSPolicy spec

[mikenairn]

What

The defaultGeo value in spec.loadBalancing.geo should be required, and it should not be possible to create a geo section without providing it with a non zero length string.

Invalid:

spec:
  loadBalancing:
    geo: {}
  routingStrategy: loadbalanced
  targetRef:
    group: gateway.networking.k8s.io
    kind: Gateway
    name: distributed-us

It should also handle better the case where the geo for whatever reason is calculated to be an empty string. Currently it will create invalid dnsNames like .klb.distributed.managed.hcpapps.net where it should be <geocode>.klb.distributed.managed.hcpapps.net.