Support filtering on null values

In the this change, https://review.openstack.org/#/c/435595/, we went for filtering out all URL params that would be generated as being like 'key=None' when generating links. This was done in this way to quickly address an immediate concern from some attributes in the schema defaulting to None needlessly. We wish to go for a more long-term, future-proof solution for this problem and leave room for actually filtering for resources that have some attribute set to 'None' or 'null'.