Where a category page lists sub-categories, ensure that the empty sub-categories have different classes to the populated sub-categories to allow different CSS styles.

Maybe use "CategoryTreeItemEmpty" instead of "CategoryTreeItem" in the DIV?
Maybe add "CategoryTreeItemEmpty" to the DIV as well as "CategoryTreeItem"?

See: https://en.wikipedia.org/wiki/Category:Infoboxes_with_unknown_parameters
This would be for all skins