Fallback to config auth if env vars are empty

When the configured environment varibales are empty, the auth should be
taken from the config file. With this, it is then possible to reference
env variables in the paket.dependencies which are used on the CI server
and still use the global file on a local machine during development.
Merge pull request #459 from yannisgu/fallbackToConfigAuth

Fallback to config auth if environment variables are empty