fix: Cannot read property 'headers' of null at Server.socket.on

lib/servers/SockJSServer.js

@@ -64,7 +64,7 @@ module.exports = class SockJSServer extends BaseServer {
   // f should be passed the resulting connection and the connection headers
   onConnection(f) {
     this.socket.on('connection', (connection) => {
-      f(connection, connection.headers);
+      f(connection, connection ? connection.headers : null);
     });
   }