Fix ceilometer parse notifications

In case of empty notification list, started_at and finished_at are
not changed and keep type int (== 0) that is passed to msec() func
that expect datetime object.

So just catch this situation.

Change-Id: I2b2ade11fee2fb7fbe88bc8fd602007d342d78e9

osprofiler/parsers/ceilometer.py

@@ -109,7 +109,7 @@ def parse_notifications(notifications):
         "info": {
             "name": "total",
             "started": 0,
-            "finished": msec(finished_at - started_at)
+            "finished": msec(finished_at - started_at) if started_at else 0
         },
         "children": _build_tree(result)
     }

tests/parsers/test_ceilometer.py

@@ -76,6 +76,17 @@ class CeilometerParserTestCase(test.TestCase):
 
         self.assertEqual(ceilometer._build_tree(test_input), expected_output)
 
+    def test_parse_notifications_empty(self):
+        expected = {
+            "info": {
+                "name": "total",
+                "started": 0,
+                "finished": 0
+            },
+            "children": []
+        }
+        self.assertEqual(ceilometer.parse_notifications([]), expected)
+
     def test_parse_notifications(self):
         samples = [
             {