Yeah, I'd agree that you can believe in Heaven and not believe in Hell. But, at least in Christianity, you have to believe in the existence of Hell. You just don't necessarily have to think it's populated by anyone. Hell's existence is only necessitated by the possibility that someone could refuse to accept God's forgiveness/love and choose to be on their own instead. In Christianity, Heaven isn't a reward/carrot, Hell a punishment/stick. Heaven is just the place where God's love is allowed into people's hearts. Hell, locked from the inside, is a place people choose to be, where God respects people's wishes to leave be left alone.
BUT, like I said, it could be empty.