fix 字典获取值问题

Src/Dlhis.ScheduledTask/TaskAssignment/Services/Implements/AssignmentService.cs

@@ -158,7 +158,7 @@ public class AssignmentService : IAssignmentService
                                 {
                                     DateTimePeriod = new DateTimePeriod
                                     {
-                                        Start = deviceUploadInfos[d] ?? DateTime.Now,
+                                        Start = deviceUploadInfos.TryGetValue(d, out var dt) ? (dt ?? DateTime.Now) : DateTime.Now,
                                         End = DateTime.Now.AddDays(1)
                                     }
                                 };