Cannot get router.external_gateway_info if it is None

Now, python-openstackclient (OSC) is using SDK to implement network functionalities. When implementing router related functionalitites, I found the following problem:

# openstack router create aaa [1]
# openstack router list --long [2]
'NoneType' object is not iterable

To reproduce this, please install the latest OSC.

The cause of this problem is: Using command [1] to create a router, no option is specified, then the router's external_gateway_info is None. But in SDK, openstack/network/v2/router.py,

class Router(resource.Resource):
    ......
    #: The ``network_id``, for the external gateway. *Type: dict*
    external_gateway_info = resource.prop('external_gateway_info', type=dict)
    ......

resource.prop() will restrict the type of external_gateway_info is dict, which has reimplemented __get__().

def __get__(self, instance, owner):
    ......
    if self.type and not isinstance(value, self.type):
            if issubclass(self.type, Resource):
                if isinstance(value, six.string_types):
                    value = self.type({self.type.id_attribute: value})
                else:
                    value = self.type(value)
    ......

It will fall into this block, and when doing type(None), here it is dict(None), it gives "'NoneType' object is not iterable" error.

The same thing won't happen to router.routes because if there is no routes in the router, router.routes is [], not None.

On OSC side, we can do nothing. So there are two solution:
1. Modify neutron to return a {}, not None, if the external_gateway_info is not set.
2. Modify SDK to check if an attr is None, do not do type() on it because self.type could be overwritten to any type.