If `post__in` or `post_parent__in` is passed to `WP_Query` as an empty array, nuke the query. Both vars are currently only checked for truthiness after which they are ignored. Setting these vars at all indicates explicit filtering being desired.

Adds unit test.

Fixes #28099.

Built from https://develop.svn.wordpress.org/trunk@28613


git-svn-id: http://core.svn.wordpress.org/trunk@28437 1a063a9b-81f0-0310-95a4-ce76da25c4cd