Ensure packages expects an array and doesn't work with undef

Commit https://github.com/luxflux/puppet-openvpn/commit/d31a0112ebeed702c821ccac878bce15a84bdb8c breaks module on Debian 7 (Wheezy) with last Puppet (from puppetlabs repositories) version (3.4.0)

Parser fails on line 36 of install.pp:
~~~
Error: ensure_packages(): Requires array given (String) at /opt/puppet/modules/openvpn/manifests/install.pp:36 on node xxx
~~~

Fixing line 36 gives another error on line 37:
~~~
Error: ensure_packages(): Requires array given (String) at /opt/puppet/modules/openvpn/manifests/install.pp:37 on node xxx
~~~

Wrapping $::openvpn::params::additional_package on any2arry doesn't worke alone causing another parser error (which might be particular to my setup):
~~~
Error: Could not find resource 'Package[]' for relationship from 'Apt::Source[non-free]' on node xxx
~~~

So playing for safe I've added an if statment checking $::openvpn::params::additional_package

I don't know if some test coverage for this case should be added