在对UITableView排序时如何处理空项目？

Checklists

ChecklistItems

Checked

Urgent

NSMutableArray *items = [NSMutableArray arrayWithArray:fetchedResultsController.fetchedObjects];
NSSortDescriptor *itemsUncheckedDescriptor = [[NSSortDescriptor alloc] initWithKey:@\"itemsUnchecked\" ascending:NO];
NSSortDescriptor *itemsUrgentDescriptor = [[NSSortDescriptor alloc] initWithKey:@\"itemsUrgent\" ascending:NO];
NSSortDescriptor *itemsCountDescriptor = [[NSSortDescriptor alloc] initWithKey:@\"checklistItems.@count\" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:itemsUncheckedDescriptor, itemsUrgentDescriptor, itemsCountDescriptor, nil];
[items sortUsingDescriptors:sortDescriptors];