Fix the return of statistic with getting no sample

In mysql scenario, when getting statistics with no sample
(this can be presented with invalid value of sample filters), ceilometer
will raise an error about NoneType.
Like other API of ceilometer, it should return an empty list.

Change-Id: I07cbc3f03f7a5e6751e6851f14ed1cdc16ee4a4c
Closes-bug: #1292322

ceilometer/storage/impl_sqlalchemy.py

@@ -734,6 +734,10 @@ class Connection(base.Connection):
             res = self._make_stats_query(sample_filter,
                                          None,
                                          aggregate).first()
+            if not res:
+                # NOTE(liusheng):The 'res' may be NoneType, because no
+                # sample has found with sample filter(s).
+                return
 
         query = self._make_stats_query(sample_filter, groupby, aggregate)
         # HACK(jd) This is an awful method to compute stats by period, but

ceilometer/tests/storage/test_storage_scenarios.py

@@ -1364,6 +1364,14 @@ class StatisticsTest(DBTestBase,
         self.assertEqual(results.sum, 18)
         self.assertEqual(results.avg, 6)
 
+    def test_with_no_sample(self):
+        f = storage.SampleFilter(
+            user='user-not-exists',
+            meter='volume.size',
+        )
+        results = list(self.conn.get_meter_statistics(f, period=1800))
+        self.assertEqual([], results)
+
 
 class StatisticsGroupByTest(DBTestBase,
                             tests_db.MixinTestsWithBackendScenarios):