Customizer header images: Fix randomizing headers.

props mcsf.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@27688


git-svn-id: http://core.svn.wordpress.org/trunk@27527 1a063a9b-81f0-0310-95a4-ce76da25c4cd
When using `meta_query` in a `WP_Query`, passing `NOT EXISTS` or `''` to `compare` should not require `value` to be set. The resulting SQL should then produce the appropriate `OR` clause for existence of non-existence after passing the query to the `$key_only_queries` stack internally.

Adds unit tests. 

Props chrisguitarguy, for the original patch.
Fixes #23268.


Built from https://develop.svn.wordpress.org/trunk@27689


git-svn-id: http://core.svn.wordpress.org/trunk@27528 1a063a9b-81f0-0310-95a4-ce76da25c4cd