Difference of nullptr

I would like to know if the difference of "2 concrete nullptr" of the same type is guaranteed to be equal to 0. I can't find anything in the C++ standard that guarantees that.

template <typename T>
std::ptrdiff_t diff() {
    T* p = nullptr;
    T* q = nullptr;
    return p - q;
}

In other words, does this code "has" to return 0?

The reason I am asking this question is that I want to implement my own vector class with

template <typename T>
class Vector {
private:
    T* data_;
    T* size_;
public:
    int size() const {
        return static_cast<int>(size_ - data_);
    }
}

and I am wondering if it is allowed to put data_ = size_ = nullptr when I construct a vector of length 0.