Not having a license does the exact opposite to what you think it does
There's a lot of "open source" projects on GitHub and other places that don't have a license attached to them. If I had to guess why, I think the people sharing these projects don't put a license on because they don't care what people do with their code. They're sharing it with the world, the world can do what it wants with it.
But that's not how copyright works - in effect, they're doing the exact opposite. Computer code is automatically copyrighted, meaning nobody can do anything with it without getting permission from the author. When you attach a license to your code, you're giving the world permission to use it.
This means, code without a license can't legally be used. Not long ago at work, I spent several days completely reimplementing from scratch something that someone had already done, because they didn't have a license on their repo, so we couldn't legally use it. They'd put it up on GitHub, so they'd obviously intended for people to use it, but nobody could.
If you have open source code without a license, it's time to add one. If you're not sure which, tl;drLegal is a great site that summarises the differences between all your options. If you want anyone to be able to use your code to do anything with no restrictions at all, use the WTFPL or CC0.
Update: It's been pointed out (thanks @lonetwin!) that there's a clause in Github's T&Cs that sounds like it gives all public GitHub projects an 'implicit' or 'default' license. However, there's a few things to note about this. One is that it sounds a lot like this clause is more so that if you use someone else's copyrighted work, they can't sue GitHub, but it still might not stop them suing you. Another is that it only says others can "view and fork", not edit, alter, distribute, use in derivative works, and all that other junk that proper licenses usually talk about. A third is that this is an agreement between the other user and GitHub—not the other user and you—which might be important. With all that in mind, I'd steer clear of making any assumptions about that clause without proper legal advice (which this isn't, because I'm not a lawyer).