Technically, all GitHub repos have at least one license. That is the GitHub license:
> If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub's functionality (for example, through forking).
> You grant us and our legal successors the right to store, parse, and display Your Content, and make incidental copies as necessary to render the Website and provide the Service.
https://help.github.com/en/articles/github-terms-of-service
