Actually, I got this wrong, the best would have been here to use null-ptr from dynamic-graph:
https://github.com/stack-of-tasks/dynamic-graph/blob/master/include/dynamic-graph/null-ptr.hh

But, otherwise in C++, the null pointer is defined by the ISO specification (§4.10/1) as
```
A null pointer constant is an integral constant expression (5.19) rvalue of integer type that evaluates to zero.
```