Nicely handle empty GITHUB_INCLUDE_REPOS

Hey @jpetazzo!

As I told you over twitter, I was having trouble getting gunsub to work down here and I ended up tracking it down to `github_include_repos` set to `['']` because of [this split](https://github.com/jpetazzo/gunsub/blob/a6051340090d6149228b7dd52beaaf612a96d02e/gunsub.py#L110) in case it is not provided and no project [would match with it](https://github.com/jpetazzo/gunsub/blob/a6051340090d6149228b7dd52beaaf612a96d02e/gunsub.py#L62-L64).

This is my first python PR ever, so apologies if somehow this could have been done in another way :-)