Fix NoneType error when calling MetricsWeigher

Given that dict comprehensions don't allow to have None as a list, we had
a problem with the Ironic gate-ironic-inspector-dsvm job because
host_state.metrics was set to None by default.

Adding a unittest to make sure we don't regress.

Closes-Bug: #1499271
Change-Id: I6e3104bbd9b9865ec2dcc724c2f56da6d0b2b687

nova/scheduler/weights/metrics.py

@@ -86,7 +86,7 @@ class MetricsWeigher(weights.BaseHostWeigher):
 
         # NOTE(sbauza): Keying a dict of Metrics per metric name given that we
         # have a MonitorMetricList object
-        metrics_dict = {m.name: m for m in host_state.metrics}
+        metrics_dict = {m.name: m for m in host_state.metrics or []}
         for (name, ratio) in self.setting:
             try:
                 value += metrics_dict[name].value * ratio

nova/tests/unit/scheduler/weights/test_weights_metrics.py

@@ -74,6 +74,15 @@ class MetricsWeigherTestCase(test.NoDBTestCase):
         self.assertEqual(expected_weight, weighed_host.weight)
         self.assertEqual(expected_host, weighed_host.obj.host)
 
+    def test_single_resource_no_metrics(self):
+        setting = [idle + '=1']
+        hostinfo_list = [fakes.FakeHostState('host1', 'node1',
+                                             {'metrics': None}),
+                         fakes.FakeHostState('host2', 'node2',
+                                             {'metrics': None})]
+        self.assertRaises(exception.ComputeHostMetricNotFound,
+                          self._get_weighed_host, hostinfo_list, setting)
+
     def test_single_resource(self):
         # host1: idle=512
         # host2: idle=1024