gh-93442: Make C++ version of _Py_CAST work with 0/NULL. (#93500)

Add C++ overloads for _Py_CAST_impl() to handle 0/NULL.  This will allow
C++ extensions that pass 0 or NULL to macros using _Py_CAST() to
continue to compile.  Without this, you get an error like:

    invalid ‘static_cast’ from type ‘int’ to type ‘_object*’

The modern way to use a NULL value in C++ is to use nullptr.  However,
we want to not break extensions that do things the old way.

Co-authored-by: serge-sans-paille