swaggerpy/client.py: Fix spurious exception when default value is 0

If a required parameter has a value that is logically False - such as 0 -
the test for the existance of the required value will always fail, even
if the user has provided a value.

This patch fixes this by testing specifically for None. If the user fails
to provide a required parameter, this will correctly throw the exception;
otherwise, it will use the value provided (even if that value is 0 or
False).