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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;
}