Add nullable to new ASCommonTableViewDelegate methods (#1796)

These were recently introduced in #1795 but were missing `nullable`
annotations on their return types.

Also, reorder the declarations to match `UITableView.h`.

Source/ASTableViewProtocols.h

@@ -78,11 +78,10 @@ NS_ASSUME_NONNULL_BEGIN
 - (nullable NSArray<UITableViewRowAction *> *)tableView:(UITableView *)tableView editActionsForRowAtIndexPath:(NSIndexPath *)indexPath API_DEPRECATED_WITH_REPLACEMENT("tableView:trailingSwipeActionsConfigurationForRowAtIndexPath:", ios(8.0, 13.0));
 - (nullable UISwipeActionsConfiguration *)tableView:(UITableView *)tableView leadingSwipeActionsConfigurationForRowAtIndexPath:(NSIndexPath *)indexPath API_AVAILABLE(ios(11.0));
 - (nullable UISwipeActionsConfiguration *)tableView:(UITableView *)tableView trailingSwipeActionsConfigurationForRowAtIndexPath:(NSIndexPath *)indexPath API_AVAILABLE(ios(11.0));
-- (UIContextMenuConfiguration *)tableView:(UITableView *)tableView contextMenuConfigurationForRowAtIndexPath:(NSIndexPath *)indexPath point:(CGPoint)point API_AVAILABLE(ios(13.0));
-- (UITargetedPreview *)tableView:(UITableView *)tableView previewForDismissingContextMenuWithConfiguration:(UIContextMenuConfiguration *)configuration API_AVAILABLE(ios(13.0));
-- (UITargetedPreview *)tableView:(UITableView *)tableView previewForHighlightingContextMenuWithConfiguration:(UIContextMenuConfiguration *)configuration API_AVAILABLE(ios(13.0));
+- (nullable UIContextMenuConfiguration *)tableView:(UITableView *)tableView contextMenuConfigurationForRowAtIndexPath:(NSIndexPath *)indexPath point:(CGPoint)point API_AVAILABLE(ios(13.0));
+- (nullable UITargetedPreview *)tableView:(UITableView *)tableView previewForHighlightingContextMenuWithConfiguration:(UIContextMenuConfiguration *)configuration API_AVAILABLE(ios(13.0));
+- (nullable UITargetedPreview *)tableView:(UITableView *)tableView previewForDismissingContextMenuWithConfiguration:(UIContextMenuConfiguration *)configuration API_AVAILABLE(ios(13.0));
 - (void)tableView:(UITableView *)tableView willPerformPreviewActionForMenuWithConfiguration:(UIContextMenuConfiguration *)configuration animator:(id<UIContextMenuInteractionCommitAnimating>)animator API_AVAILABLE(ios(13.0));
-
 #endif
 - (BOOL)tableView:(UITableView *)tableView shouldIndentWhileEditingRowAtIndexPath:(NSIndexPath *)indexPath;