On the first request served by a PHP process, a Drupal 6 or 7 hook that was invoked with module_invoke() before it had been invoked by module_invoke_all() could have its module name reported as an empty string. This has been fixed.
On PHP 7.0 and 7.1, a Drupal 6 or 7 hook invoked via module_invoke_all() could report the module name as an empty string in some situations. This has been fixed.