Fixed https://github.com/example42/puppet-network/issues/28.

This should fix the problem with empty address statements on debian systems. The same thing (empty IPADDR options) is likely to occur on Redhat/SuSE systems as well. I do not have a any RedHat or SuSE boxes available right now, so I don't know whether it is in fact a problem or merely a cosmetic issue on these systems.