Only use the GITHUB_API_TOKEN if it's not empty

Unfortunately the Ruby ENV['foo'] returns true regardless or not if the value is empty or not.

I was having some issues with a terminal session where I purposefully set all env variables to blank for when I'm screen sharing and then puppet-librarian kept failing with a 401. Looking it up it was because it was calling `https://api.github.com/foo?page=1&per_page=100?access_token=`

I put in a Puts to alert the user, but can remove if needed