Handle Blank Response From Web Server

At present, I was facing some sort of weird problem. I turn off my internet connection to put some handling code over their. I thought it will return me some error code but its just giving me blank response rather than showing exception. I got following debug output when I print more details on screen.

Basically I want to show dialog box when there is no internet connection. But how to handle this situation!!!

Because there is no json response from server side then also there is some bytes I am receiving from server. Here is my code:

 Dictionary<string,string> headerDisc = new Dictionary<string, string> ();
         headerDisc.Add ("Api-Key", "You API Key");
 
         WWW www = new WWW (GameConstants.CONTESTANT_LIST_BASE_URL, new byte[] { (byte)0 }, headerDisc);
         yield return www;
 
         if (www.error == null) {
 
             Debug.Log ("bytes: " + www.bytes.Length);
             Debug.Log ("size: " + www.size);        
             Debug.Log ("length: " + www.text.Length);
             Debug.Log ("Data: " + www.text);
 
             if (www.text.Length <= 0) {
                 AppManager.Instance.DialogMessage = "No Server Response Found!";
                 Camera.main.SendMessage ("ActivateDialogBoxPanel", true, SendMessageOptions.DontRequireReceiver);
             } else {
 
                 JSONObject jsonObj = new JSONObject (www.text);
                 JSONObject messageObj = jsonObj [TAG_MESSAGE];
 
                 string successValueStr = jsonObj [TAG_SUCCESS].ToString ();
                 if (successValueStr.Equals (VALUE_TRUE)) 
                     // success
                 else 
                     // fail
             }
         } else {
             Debug.Log ("Error: " + www.error);
             AppManager.Instance.DialogMessage = "Error:" + www.error;
             Camera.main.SendMessage ("ActivateDialogBoxPanel", true, SendMessageOptions.DontRequireReceiver);
         }

Please give me some suggestion in this. If you want some more information then I am available.