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.


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