I believe I understand the meaning of "Nul n'aurait pu le dire", but I'm unsure about the grammar.
First of all, am I correct in thinking that aurait pu is the plus-que-parfait?
Second, why is there nothing to complete the negation? I would have expected something like:
Nul n'aurait pas pu le dire
In other words, why is there no pas (or something similar) to complete it?