Python: fixed None usage as Id in session.exec_()

Check must directly compare PyObject* to Py_None singleton, because other method
(checking object equality to python::api::object()) involves user defined
comparing/equality operators.
This is the case with elliptics.Id which has __cmp__ method, which
cannot accept None as an argument.
Merge pull request #334 from ijon/fix-python-none-as-exec-id

Python: fixed None usage as Id in session.exec_()