I think that's an interesting point. I suppose I was thinking that nihilism, at least in the way its typically discussed, holds not that doing nothing is rational but, rather, that no goals are rational (a subtle difference, perhaps). This, in my opinion, might equate with all goals being equally possible. But, as you point out, if all goals are equally possible the agent might default to doing nothing.

One might put it like this: the agent would be landed in the equivalent of a Buridan's Ass dilemma. As far as I recall, the possibility that a CPU would be landed in such a dilemma was a genuine problem in the early days of computer science. I believe there was some protocol introduced to sidestep the problem.