Fix params is None issue

When user call `cursor.execute('sql statement', None)` directly, ` return map(self._quote_expr, params) ` would crash. See screenshot below.
![screen shot 2013-05-07 at 6 02 57 pm](https://f.cloud.github.com/assets/1174439/470614/db64992e-b6fd-11e2-9623-d874c3a5d390.png)