Don't iterate over `$wp_query->posts` in `update_post_thumbnail_cache()` if it is empty. Adds unit tests.

Props SergeyBiryukov, for the original patch.
Fixes #26321.
 


git-svn-id: https://develop.svn.wordpress.org/trunk@27166 602fd350-edb4-49c9-b593-d223f7449a82