In `WP_Meta_Query`, interpret 'value' correctly when used with EXISTS/NOT EXISTS.

As in earlier versions, EXISTS with a value is equivalent to '=', while NOT
EXISTS should always ignore 'value'.

Props barrykooij.
Fixes #30681 for trunk.
Built from https://develop.svn.wordpress.org/trunk@30846


git-svn-id: http://core.svn.wordpress.org/trunk@30836 1a063a9b-81f0-0310-95a4-ce76da25c4cd