make critpath file optional
    
    Don't crash if critpath file is missing or is empty. This is a
    preparation for shutting down pkgdb where we pull critpath from
    currently. Critpath is also available in PDC, but that's also going
    away. New location is currently unknown. Since we don't use critpath in
    any task at the moment, let's keep this functionality but don't fail
    when the file is not present.
    
    Merges https://pagure.io/taskotron/taskotron-trigger/pull-request/53