处理size为null的错误。

TEAMModelOS.SDK/Models/Cosmos/Teacher/Research/TeacherFile.cs

@@ -22,7 +22,7 @@ namespace TEAMModelOS.SDK.Models
         /// <summary>
         /// 文件大小
         /// </summary>
-        public long size { get; set; }
+        public long? size { get; set; }
         /// <summary>
         /// 视频的时长
         /// </summary>
@@ -52,7 +52,7 @@ namespace TEAMModelOS.SDK.Models
         public string nodeId { get; set; }
         public string url { get; set; }
         public string type { get; set; }
-        public long size { get; set; }
+        public long? size { get; set; }
     }
     public class FileAbility { 
         /// <summary>