Do not query the DB if QoS policy ID is None
Do not query the DB if QoS policy ID is None

When retrieving the QoS policy rules, if the QoS policy ID passed is
None, do not query the DB and return an empty rule list for this
non existing QoS policy.

Change-Id: Idccbd22a1dc9bef81b2cdae0a597c1a587a60ae0

openstack/neutron,openstack/neutron,mahak/neutron,openstack/neutron,mahak/neutron,mahak/neutron