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'.

Merges [30846] to the 4.1 branch.

Props barrykooij.
Fixes #30681

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Improve text for the DFW admin pointer. Remove the concatenation.
Props kpdesign, ianmjones.

Merges [30837] to the 4.1 branch.

See #30671.

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