Ensure that terms with a post count of `0` are not returned in `wp_count_terms()` when `hide_empty => true`. Adds unit test which ensures that `wp_count_terms()` returns `0`.

Props markjaquith.
Fixes #15919.



git-svn-id: https://develop.svn.wordpress.org/trunk@25551 602fd350-edb4-49c9-b593-d223f7449a82