Enable empty log file

Problem/Motivation

It can be useful to set the module to only log in some situations.
For example, to have a condition in settings.php that only applies for drush commands - this can be useful on Acquia hosting - see https://docs.acquia.com/cloud-platform/manage/cron/advanced-cron

However, the module assumes that the log_filename config value will always be a path to a file.