Kill the query in the following edge case: `post_type => 'any'` but `exclude_from_search => false` returns no valid post types. Adds unit tests.

Props mitchoyoshitaka.
Fixes #19198.



git-svn-id: https://develop.svn.wordpress.org/trunk@25239 602fd350-edb4-49c9-b593-d223f7449a82