Added unless modifier to prevent displaying no upstream servers.

Even though #print_more will display nothing since ''.split("\n").each()
iterates 0 times, it will still be called without this modifier which is
unnecessary.

extensions/dns/api.rb

@@ -52,7 +52,7 @@ module BeEF
             dns.run(:upstream => servers, :listen => interfaces)
 
             print_info "DNS Server: #{address}:#{port} (#{protocol})"
-            print_more upstream_servers
+            print_more upstream_servers unless upstream_servers.empty?
           end
 
           # Mounts the handler for processing DNS RESTful API requests.