empty string no longer equivalent to undef in puppet 4.0>

[Jakeii]

Seen here https://tickets.puppetlabs.com/browse/PUP-1037

this now breaks here https://github.com/netmanagers/puppet-fail2ban/blob/master/manifests/init.pp#L501 beause source_dir default is '', and file source param can't be ''

workaround was to explicitly set source_dir to false.