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.



git-svn-id: https://develop.svn.wordpress.org/trunk@27689 602fd350-edb4-49c9-b593-d223f7449a82