Why is "operator delete" invoked when I call "delete" on a null pointer?

While reading answers to this question I noticed that answers (this for example) imply that operator delete can be called even when delete statement is executed on a null pointer.

So I wrote a small snippet:

class Test {
public:
    void* operator new( size_t ) { /*doesn't matter*/ return 0; }
    void operator delete( void* ptr ) {
        ptr; //to suppress warning and have a line to put breakpoint on
    }
};

int main()
{
    Test* ptr = 0;
    delete ptr;
}

and - surprisingly for me - Test::operator delete() is invoked with ptr holding a null pointer.

As I understand it operator new allocates memory and operator delete returns memory to the allocator. If I call delete statement on a null pointer it means there was no object behind the pointer and there's no memory to return to the allocator.

delete statement includes invoking a destructor. When I pass a null pointer the destructor is surely not invoked - C++ takes care of that. Then why is operator delete invoked in this case?