In Racket and Scheme, false is the only 'falsy' value, with everything else evaluating 'truthy', which is nice since you don't have to worry about the specifics of the return type.

I think the bigger issue with null or None, though, is not necessarily using it as a return type (where that makes sense, and can be enforced by a type system) but when objects are mutated to the value of null. The bigger issue there is with mutation and side-effects, but having null there as a convenient short cut can be a foot gun that leads to lots of errors.