Users: Set the default value of COOKIE_DOMAIN to an empty string.

This matches the type expected by the `setcookie()` function for the `$domain` parameter, and resolves a fatal error if `strict_types` is enabled.

Reference: [https://www.php.net/setcookie PHP Manual: setcookie()].

Follow-up to [2725], [6434], [12732], [13062].

Props kmvan, rajinsharwar, jrf, desrosj, Cybr, nicolefurlan, oglekler, hellofromTonya, kirasong, chaion07, mukesh27.
Fixes #46550.

git-svn-id: https://develop.svn.wordpress.org/trunk@58011 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/default-constants.php

@@ -325,9 +325,10 @@ function wp_cookie_constants() {
 
 	/**
 	 * @since 2.0.0
+	 * @since 6.5.0 The value has changed from false to an empty string.
 	 */
 	if ( ! defined( 'COOKIE_DOMAIN' ) ) {
-		define( 'COOKIE_DOMAIN', false );
+		define( 'COOKIE_DOMAIN', '' );
 	}
 
 	if ( ! defined( 'RECOVERY_MODE_COOKIE' ) ) {