Treat Filter as Dict in get_policies

Currently QoS plugin's get_policies doesnt check if the filter
is a dict or not, though similar checks exist in get_rule_types()
and get_policy_rules().
This patch adds the check so that if any consumer passes filters=None
then it is converted to a dict.

TrivialFix
Partial-Bug: #1711165

Change-Id: Id50674c44cd3720dcd8fd6d66361d937599f5992

neutron/services/qos/qos_plugin.py

@@ -262,6 +262,7 @@ class QoSPlugin(qos.QoSPluginBase):
 
         :returns: QosPolicy objects meeting the search criteria
         """
+        filters = filters or dict()
         pager = base_obj.Pager(sorts, limit, page_reverse, marker)
         return policy_object.QosPolicy.get_objects(context, _pager=pager,
                                                    **filters)