The conjecture about dynamically typed languages was regarding the cultural effect of the popularity non-null-safe static type systems on the current practice of coding in dynamic programming languages. It wasn't a conjecture about the structure of the type systems in those dynamically typed languages.
I find that even less plausible. I don't suspect that the null reference practices in, say Java, have any influence on the way people work in Lisp.
Optional parameters in Lisp take on a value of nil when the argument is omitted (unless a different default is specified). Various search functions return nil when an item is not found: find, member, gethash, ...
The practices around these conventions may, at times, resemble work in static languages with null references, but there is no cause-and-effect there.
