fix 获取日志最后一条处理

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

@@ -318,6 +318,8 @@ public class AssignmentService : IAssignmentService
                 var logInfo = logInfos.FirstOrDefault(x => x.DeviceUniqueNo == item);
                 result.Add(item, logInfo?.RaiseDateTime);
             }
+
+            return result;
         }
 
         return new Dictionary<string, DateTime?>();