fix nullable.value问题,改为使用修改时间

Src/Dlhis.Basic/Organization/Services/Implements/GainfoUpdateService.cs

@@ -312,7 +312,7 @@ public class GainfoUpdateService : IGainfoUpdateService
                         {
                             try
                             {
-                                var resignationDate = DateOnly.FromDateTime(gainfo.ResignationDate!.Value);
+                                var resignationDate = DateOnly.FromDateTime(gainfo.ModifiedTime);
 
                                 var workGridSchedules = await _dbContext.Queryable<WorkGridScheduleEntity>().Where(x => x.UserCode == gainfo.Code)
                                     .Where(x => x.Date >= resignationDate)