Change syntax to be 7.4 compat

Author: topher1kenobe

inc/user-notification/namespace.php

@@ -21,7 +21,7 @@ function bootstrap() {
  * @param string $content The current version or update notification.
  * @return string
  */
-function notify_users( string|null $content = '' ) : string {
+function notify_users( ?string $content = null ) : string {
 	$message = sprintf(
 		// translators: 1) Fair PM URL, 2) AspirePress URL.
 		__( 'Updates served from the <a href="%1$s">FAIR Package Manager</a> and <a href="%2$s">AspirePress</a>', 'fair' ),