Handle None values in parse_value for Time and Date Fields
Handle None values in parse_value for Time and Date Fields

Have parse_value return None when the passed value is None. This allows
DateField and TimeField to return None rather than fail with a TypeError
when the original value is None (or non-existant). This can happen when
e.g. serializing an object where the Date (or Time) Field is optional
(required=False).

beregond/jsonmodels