no api for unregistered device

src/common.rs

@@ -960,6 +960,9 @@ pub fn get_custom_rendezvous_server(custom: String) -> String {
 
 #[inline]
 pub fn get_api_server(api: String, custom: String) -> String {
+    if Config::no_register_device() {
+        return "".to_owned();
+    }
     let mut res = get_api_server_(api, custom);
     if res.ends_with('/') {
         res.pop();