change_user method doesn't allow None values

change_user() method of connection object doesn't allow to pass None values, for e.g. empty password:

>>> import mariadb
>>> conn=mariadb.connect(user="root")
>>> conn.change_user("root", None, None)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: argument 2 must be str, not None