No, generally atheists aren't necessarily nihilists.
Just because an atheist does not believe in a god does not mean they don't have any beliefs, period. You'll find that many atheists believe that:
- Working hard is good.
- The world is beautiful.
- Helping others is also good.
- Science is the best tool we have for figuring out reality.
- Educating the next generation should be a priority.
- Cooperating with others, and building a stable society is good.
... and so on.
It is a bit silly to say that an atheist would believe in nothing - just not an all-powerful universe-creating guy.