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

wp-includes/meta.php

@@ -1419,6 +1419,17 @@ class WP_Meta_Query {
 					$where = $wpdb->prepare( '%s', $meta_value );
 					break;
 
+				// EXISTS with a value is interpreted as '='.
+				case 'EXISTS' :
+					$meta_compare = '=';
+					$where = $wpdb->prepare( '%s', $meta_value );
+					break;
+
+				// 'value' is ignored for NOT EXISTS.
+				case 'NOT EXISTS' :
+					$where = '';
+					break;
+
 				default :
 					$where = $wpdb->prepare( '%s', $meta_value );
 					break;