ensure nil is returned by custom fact when no runners are configured

Use same method like i used here : https://github.com/voxpupuli/puppet-openvpn/blob/master/lib/facter/easyrsa.rb#L43

And so at https://gitlab.adullact.net/adullact/puppet-gitlabrunner/blob/master/manifests/init.pp#L42 use a case statement like :

$_defined_runners_in_facts = $facts['gitlabrunners'] ? {
  undef   => [],
  default => $facts['gitlabrunners'],
}