The region table doesn't allow for nullable descriptions [1] . The catalog Manager checks if region['description'] is set in the request and if the user hasn't provided a description for the region, the Manager will set it to an empty string [2]. If the user creates a region with a description and then later tries to update the description to be None, or an empty string, the request will fail because validation against the description field will fail.
Invalid input for field 'description'. The value is 'None'.
The user should be able to pass None, or null in json, to Keystone in a region request. Region description are documented as being optional.
[1] https://github.com/openstack/keystone/blob/2d829b4d9a886909735daa0f8a9419c8ba8d3f87/keystone/common/validation/parameter_types.py#L40-L42
[2] https://github.com/openstack/keystone/blob/2d829b4d9a886909735daa0f8a9419c8ba8d3f87/keystone/catalog/core.py#L103-L106
@dstanek had a patch sitting in review for this that looks good, but I'm assigning to him since I didn't write the code :)