Functional languages typically don't have a concept of null for the simple reason that it's always a special case. If you strictly follow functional principles, each input must have a workable output—but what is null? Is it an error? Or within normal operating parameters, but a negative result?
As a legacy feature, null has been around since C/C++, when a pointer could actually point to the (invalid) address, 0. However, many new languages try to move away from that. Rust does not have null, and no return value as a normal case with the Option type. The case of error is covered by the Result type, to which we dedicated an entire chapter, Chapter 5, Handling Errors and Other Results.
