GUACAMOLE-250: Require callback - calling exportState() makes no sense otherwise.

guacamole-common-js/src/main/webapp/modules/Client.js

@@ -196,8 +196,7 @@ Guacamole.Client = function(tunnel) {
             }
 
             // Invoke callback now that the state is ready
-            if (callback)
-                callback(state);
+            callback(state);
 
         });