If a repository has no license, then all rights are reserved and it is not Open Source or Free. You cannot modify or redistribute this code without explicit permission from the copyright holder.
I'm unsure of the legal implications of actually pulling the source local and building/using the software privately though. Perhaps someone else can chime in on that.
From GitHub's licensing help page:
You're under no obligation to choose a license. It's your right not to
include one with your code or project, but please be aware of the
implications. Generally speaking, the absence of a license means that
the default copyright laws apply. This means that you retain all
rights to your source code and that nobody else may reproduce,
distribute, or create derivative works from your work. This might not
be what you intend.
Even if this is what you intend, if you publish your source code in a
public repository on GitHub, you have accepted the Terms of Service
which do allow other GitHub users some rights. Specifically, you allow
others to view and fork your repository.
If you want to share your work with others, we strongly encourage you
to include an open source license.
In short, the only thing you can safely assume is that you have no rights to do anything at all with this code. In the particular case of GitHub, you can fork the repository and view the code, but nothing more.
Being it's entirely possible that the owner doesn't realize any of this, I left a comment on the repo alerting them to the situation and pointing back to this Q & A.