This is great work!

The problem is that ast2obj_object translates NULL values to Py_None,
but obj2ast_object doesn't translate that back. This definition fixes
your testcase:

static int obj2ast_object(PyObject* obj, PyObject** out, PyArena* arena)
{
    if (obj == Py_None)
        obj = NULL;
    Py_XINCREF(obj);
    *out = obj;
    return 0;
}