feat: use name server reply (SOA) if record answer is empty

If the answer is empty digs will display nothing. Instead of this
behavior, digs will show the default reply of DNS name server which
is a SOA record.

Other DNS command-line client such as `dig` and `dog` also has this
behavior.

src/main.rs

@@ -71,6 +71,16 @@ fn main() {
                         res.rdata().to_string().bold()
                     );
                 }
+                for res in res.name_servers() {
+                    let rr_type = res.rr_type().to_string().green().bold();
+
+                    println!(
+                        "  {0: <15} {1: <15} {2: <10}",
+                        rr_type.to_string(),
+                        res.name().to_string().blue(),
+                        res.rdata().to_string().bold()
+                    );
+                }
             }
         }
     }