What advantages does having a null-able string type confer to a programming language?
At the risk of starting a religious war, I will express my opinion that there are no advantages to having nullable types. These should always be replaced by a sum type such as Ocaml option or Haskell Maybe (paired with sane deconcstructors for such types that force the programmer to always consider both possibilities).

The main reason for this is that null pointers, null objects, and the like are responsible for many, many bugs and countless headaches.