Support spec.rules[0].host with null value

[darkn3rd]

When spec.rules[0].host is absent or set to null, it will default to *. This is used when only one service, such as Dgraph Alpha, is used. Instead of an assigned FQDN for the hostname, the IP address of the load balancer will be used.

This is definitely, not a recommended practice, but for testing or other edge cases, this could be used. The helm install should not error when the host gets set to null through global.alpha_hostname, global.ratel_hostname, global.alpha_grpc_hostname, alpha.ingress.hostname, ratel.ingress.hostname.