Hmm, oh! C++ comes to mind. Of course, there is such a thing as null, but in:
void fun(string x) // std::string
{
}
x cannot be null. The reference semantics (like a copy of a string sharing the data with the original) is an implementation detail/optimization encapsulated inside what appears to be a value type.
The tools are there in C++ to create ideal types for your problem domain which just look like value types that have no null domain value (or any other value you don't want), and standard strings are like this.
Pointers to std::string are almost never required, though. A pointer to std::string is not something you have to use to write a string handling C++ program or module; and such a pointer p is itself not a string, *p is (if p is non-null and valid).
About the only time you would need a pointer to std::string when calling some C API that takes a callback function with a void * context, and you'd like that context to be a std::string. Then you might take the address of string object to pass to that API. (That pointer would likely never be null, but could go bad due to lifetime mismanagement.)
Most other uses of such a thing would be unidiomatic. Whereas, in some languages, string references that can be null are foisted on programmers as the standard, idiomatic string representation. That's a big difference.
The tools are there in C++ to create ideal types for your problem domain which just look like value types that have no null domain value (or any other value you don't want), and standard strings are like this.