Are 'active' and 'staging' required in config_directories?
The default.settings.php file contains the following comment:


  The staging directory is used for configuration imports; the active directory is not used by default, since the default storage for active configuration is the database rather than the file system (this can be changed; see "Active configuration settings" below).


In short:


The staging directory is used only when importing configuration from the UI
The active directory is not used by default from Drupal 8, which saves the configuration data in the database, like Drupal 7 did with the persistent variables


In both the cases, I would expect Drupal to correctly handle the case the global $config_directories is not set.