I think it's silly not to allow it. And the above shows an example where it would be useful. 

And by the way, in mathematics nullary functions are a real thing. It's just a function f: {\*} -> B, where {\*} is a singleton set, i.e., the 0-th power of any nonempty set. They are used, e.g., to provide a uniformity in the definition if an algebra in the universal sense.