In any country that's a signatory to the Berne Convention, copyright -- or stronger -- is the default as soon as something is created. If you completely ignore the subject, all your work is copyrighted to you (or to your employer in many cases), and anyone who copies it to use or improve it is in breach of your copyright.
GitHub does not include any useful default licensing terms in its terms of service; the most likely scenario is that any use of the copyrighted material in one of those no-license projects is formally a breach of copyright. Under copyright law, code without a license cannot be legally shared, as the default for copyrighted materials is that all rights are reserved.
See GitHub needs to take open source seriously: