Fix check_database_version() return type: void|WP_Error → WP_Error|null

Add explicit return null when the version check passes.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: westonruter

src/wp-includes/class-wpdb.php

@@ -3998,7 +3998,7 @@ public function close() {
 	 * @since 2.5.0
 	 *
 	 * @global string $required_mysql_version The minimum required MySQL version string.
-	 * @return void|WP_Error
+	 * @return WP_Error|null
 	 */
 	public function check_database_version() {
 		global $required_mysql_version;
@@ -4009,6 +4009,8 @@ public function check_database_version() {
 			/* translators: 1: WordPress version number, 2: Minimum required MySQL version number. */
 			return new WP_Error( 'database_version', sprintf( __( '<strong>Error:</strong> WordPress %1$s requires MySQL %2$s or higher' ), $wp_version, $required_mysql_version ) );
 		}
+
+		return null;
 	}
 
 	/**