Avoid retrieving ports if network list is empty

In ``QoSPlugin._get_ports_with_policy``, if the network IDs list is
empty, the SQLAlchemy throws the following warning:
  SELECT statement has a cartesian product between FROM element(s)
  "subports_1", "ports", "portnumaaffinitypolicies_1",
  "testportextensions_1", "portuplinkstatuspropagation_1",
  "portdataplanestatuses_1", "standardattributes_2",
  "portdeviceprofiles_1", "ml2_port_bindings_1",
  "portsecuritybindings_1", "portdnses_1",
  "securitygroupportbindings_1", "qos_network_policy_bindings_1",
  "qos_port_policy_bindings_1", "trunks_1",
  "standardattributes_1" and FROM element "networks".  Apply
  join condition(s) between each element to resolve.

This patch avoids this query by checking the network IDs list. If the
list is empty, the expected port list will be too. This is also a
small optimization because we are skipping the port query.

This patch is also applying the same logic to the second query in this
method.

Closes-Bug: #2018000
Change-Id: Ia5380bc78cc1d0136e11cc4692069279419e285e

neutron/services/qos/qos_plugin.py

@@ -323,7 +323,7 @@ class QoSPlugin(qos.QoSPluginBase):
     def _get_ports_with_policy(self, context, policy):
         networks_ids = policy.get_bound_networks()
         ports_with_net_policy = ports_object.Port.get_objects(
-            context, network_id=networks_ids)
+            context, network_id=networks_ids) if networks_ids else []
 
         # Filter only this ports which don't have overwritten policy
         ports_with_net_policy = [
@@ -333,7 +333,7 @@ class QoSPlugin(qos.QoSPluginBase):
 
         ports_ids = policy.get_bound_ports()
         ports_with_policy = ports_object.Port.get_objects(
-            context, id=ports_ids)
+            context, id=ports_ids) if ports_ids else []
         return list(set(ports_with_policy + ports_with_net_policy))
 
     def _validate_create_port_callback(self, resource, event, trigger,