API: Autofill Service.Address with Node.Address if Service.Address is empty

#### Feature Description

Whenever the Consul API reports a `ServiceAddress` or `Service.Address`, if this value is blank/empty, default to the address of the Node instead of leaving the value blank.

We currently document that a blank value is expected behaviour in the catalog endpoint documentation, although changing this could improve DX and is therefore a feature request.

https://www.consul.io/api/catalog.html#serviceaddress

> ServiceAddress is the IP address of the service host — if empty, node address should be used

#### Use Case(s)

Make it easier for API consumers/clients to retrieve the IP address of a service, even when there is no need for the registration of the service to provide an IP.

Also see:

https://github.com/hashicorp/consul-template/issues/587#issuecomment-232456696
https://github.com/hashicorp/consul/issues/4579

@mkeeler this is re: offline chat