轨迹标记编辑输入参数，字符串类型加上 ? 可空

src/SJZY.Track.Application/Dtos/TrackFlags/EditTrackFlagInput.cs

@@ -26,12 +26,12 @@ namespace SJZY.Track.Application.Dtos.TrackFlags
         /// <summary>
         /// 标记
         /// </summary>
-        public string Flag { get; set; }
+        public string? Flag { get; set; }
 
         /// <summary>
         /// 代码
         /// </summary>
-        public string Code { get; set; }
+        public string? Code { get; set; }
 
         /// <summary>
         /// 是否完结