Fix error on initialization with no symbols

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

src/threads/data/websocket.rs

@@ -128,6 +128,11 @@ async fn handle_message(
     sink: Arc<Mutex<SplitSink<WebSocketStream<MaybeTlsStream<TcpStream>>, tungstenite::Message>>>,
     message: Message,
 ) {
+    if message.symbols.is_empty() {
+        message.response.send(()).unwrap();
+        return;
+    }
+
     match message.action {
         Some(Action::Subscribe) => {
             let (pending_subscriptions, receivers): (Vec<_>, Vec<_>) = message