Fix to not use empty IN clause

When an empty IN clause is used with sqlalchemy, a warning is
printed:

 SAWarning: The IN-predicate on "assignment.actor_id" was invoked
 with an empty sequence. ...

With this fix, these warnings should no longer be printed.

The tests are changed to convert these warnings into errors.

Change-Id: I58c7e5f11fdac9bbc1e9a970ce690ea113e2bd95
Closes-Bug: #1404354

keystone/tests/unit/test_core.py

@@ -13,7 +13,9 @@
 #   under the License.
 
 import sys
+import warnings
 
+from sqlalchemy import exc
 from testtools import matchers
 
 from keystone.openstack.common import log
@@ -35,3 +37,8 @@ class TestTestCase(tests.TestCase):
         self.assertThat(
             lambda: LOG.warn('String %(p1)s %(p2)s', {'p1': 'something'}),
             matchers.raises(tests.BadLog))
+
+    def test_sa_warning(self):
+        self.assertThat(
+            lambda: warnings.warn('test sa warning error', exc.SAWarning),
+            matchers.raises(exc.SAWarning))