如何处理JSON中为空的变量

在我的JSON文件中，有一个数组（租金）。在此数组中，有一个变量，序列，可以为null或字符串。如果串行变量为null，我想遍历数组（租金），我想继续循环，但是当它不为null时，我想保存它。但是它不以这种方式表现。当获得“序列”时，数组停止循环。

这是我的代码：

    public static string sendRequest(string url, string method) {
            try
            {
                // Set reqiued information for api
                var httpWebRequest = (HttpWebRequest)WebRequest.Create(main_url + url);
                httpWebRequest.ContentType = "application/x-www-form-urlencoded";
                httpWebRequest.Headers.Add("Authorization", "Bearer " + Current_Access_Token);
                httpWebRequest.Accept = "application/json;v=1";
                httpWebRequest.Method = method;

                // Get adn return responses
                var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
                string respStr = new StreamReader(httpResponse.GetResponseStream()).ReadToEnd();
                return respStr;
            }
            catch (Exception e)
            {
                MessageBox.Show("No responses");
            }
            return "";
        }


public static void GetRentGames()
        {
            string resp = sendRequest("api.php/currentCustomer/rents", "GET");
            JObject jObject = JObject.Parse(resp);
            JArray Juser = (JArray)jObject["response"]["data"]["rents"];
            //Console.WriteLine(resp);
            foreach(var i in Juser)
            {
                var matchGame = ApiConnector.rentGame.Find(x => x.name == i["item"]["name"].ToString());

                if(matchGame == null)
                {
                    //name and id works just fine. it is the serial i can't get, and I think because the serial is null?
                    Console.WriteLine(i["serial"].ToString());
                    var Game = new RentGame(i["item"]["name"].ToString(), i["id"].ToString(), i["serial"].ToString());
                    rentGame.Add(Game);
                    Console.WriteLine("Add rent game " + Game.name);
                }

            }

        }

JSON文件-第一个索引：

"response":{
        "code":200,
        "status":"success",
        "data":{
            "rents":[
                {
                    "id":"34414",
                    "start_date":"2015-12-08",
                    "download_url":null,
                    "serial":null, ...