Patch: "empty" categories are not shown in filtered navigation

The patch addresses the issue with anchor categories, which were not displayed in layered navigation, if they directly had no products.

The reason - usage of wrong table, which is not changed. The catalog_category_product_index must be used instead.

This Patch may lead to performance degradation - as per each category in layered filter it makes one additional request to MySQL. To be addressed in future releases.