python: return error if PyObject obj is NULL for unwrapper helper functions

python: return error if PyObject obj is NULL for unwrapper helper functions

The result is indeterminate for NULL argument to python
functions as follows. It's better to return negative value in
these situations.

PyObject_IsTrue will segfault if the argument is NULL
PyFloat_AsDouble(NULL) is -1.000000
PyLong_AsUnsignedLongLong(NULL) is 0.000000