Avoid AttributeError: SecurityGroup error handle

On the project/access_and_security/security_groups/<detail> page, some
errors inside the
openstack_dashboard/dashboards/
project/access_and_security/security_groups/views
DetailView._get_data method are still resulting in a None being
returned. This causes an error in the DetailView.get_data method as
NoneTypes clearly don't have a .rules attribute.

I'm not sure why the error is sneaking through the exceptions.handle
above, but they most definitely are. Perhaps Ana Krivokapic's changes
here: https://review.openstack.org/#/c/84109/ may eventually solve this,
but it's unclear.

In the mean time, and potentially correct overall, if a None is
returned, the get_data method should return an empty list instead of
throwing an exception.

Change-Id: Ibc3bbb8c1302fca0c8a134abee8b7c649ad8c315
Closes-Bug: 1312192
Merge "Avoid AttributeError: SecurityGroup error handle"