plugin/etcd: Filter empty host field by qtype (#2499)

When a query, different from a TXT lookup is performed, all services
with a missing `Host` field should be filtered out, as these otherwize
cause a line in the answer section with a single dot (`.`) as the
result. This behavior manifests for example when a TXT record is present
on a domain, eg. an A or SRV lookup is performed on said domain.

If there are no services containing a `Host` field, a `NODATA` response
should be given. If there are other Services, these alone should be
returned for the query.

Filter any service that has an empty Host field from all lookup types
other than TXT to solve this issue. At the same time the check for empty
`Text` fields in TXT queries are also moved to the same check in the
etcd ServiceBackend.

plugin/backend_lookup.go

@@ -331,9 +331,6 @@ func TXT(b ServiceBackend, zone string, state request.Request, opt Options) (rec
 	}
 
 	for _, serv := range services {
-		if serv.Text == "" {
-			continue
-		}
 		records = append(records, serv.NewTXT(state.QName()))
 	}
 	return records, nil