I am working on the web application which provides newsletters for users. These newsletters are based on interests selected by users on their profiles. If user does not select anything he will just receive the general newsletter, but choosing his preferences would make the newsletter more personal. Does anyone can suggest what to do: to forbid users saving empty profiles or allow them to do this?
As Brian said, whether or not users would be allowed to save an empty profile is probably a business decision.
As for whether they should fill out their profile, it's a double edged sword. Not filling out your profile hurts your user by giving them non-personalized newsletters, while forcing users to fill in a profile could frustrate users and cause a hit to conversion rates. Never underestimate the importance of a single added step, like the infamous $300 million button, where a required registration form cost a major e-commerce site theoretically $300 million because there was an added step to registration.
Rather than force profile creation, make sure you're selling the benefits of the profile. Don't just say "hey, fill this out" but make a clear header on the page asking them to fill out their profile which explains why the customization is good for them. Make sure creating an empty profile isn't the default option. Make the option to continue without a profile less obvious, but don't remove it.