Skip apt-add even if .list file is empty

Change to skip apt-add if .list file is present, even if it's empty.

On Elementary OS Luna, adding e.g. git-core/ppa creates `git-core-ppa-precise.list` containing the actual data, and an empty file called `git-core-ppa-luna.list` as a placeholder. This makes `apt::ppa` execute on every Puppet run. Wouldn't have thought allowing an empty file would break anything, but happy to be corrected.