Smart Search filter should/must allow to select empty categories and its child.
At themoment It's possible to select only categories that are already indicized and have already at least an article, but if articles are inserted by many users, it's "impossible" to update the filter every time a category is filled.
It is bettere that a user can choose a category independently if it has an article or not, bettere if selecting a category, all child categories are automatically selected