Without an external source of meaning the Stoic justification for ethics crumbles, because it would not be rational to work for something that has no meaning. The rational thing to do would be to work for our own happiness and of those close to us. But there would be no justification for cultivating Virtue or sacrificing our interest for some distant goal.
I am not a nihilist, however, because I do believe that the Universe has meaning. What we need to do is to update that idea according to science and not attack the outdated concept of deity that the Stoics had.