Don't return links if subscriptions are empty

The commit I08dd0f675bcd25cd63e17a0aa9779895bd401e32 try to fix
the links format when a queue's subscriptions are empty. But it
doesn't work. Because the "results" which storage returns will
never be None.

This patch fix the nit and also add a test to ensure it works.

Change-Id: Ib7fb0b6bfccc5e913ff618257f07903f692201f1

zaqar/tests/unit/transport/wsgi/v2_0/test_subscriptions.py

@@ -176,6 +176,19 @@ class TestSubscriptionsMongoDB(base.V2Base):
     def test_list_works(self):
         self._list_subscription()
 
+    def test_list_empty(self):
+        resp = self.simulate_get(self.subscription_path,
+                                 headers=self.headers)
+
+        self.assertEqual(falcon.HTTP_200, self.srmock.status)
+
+        resp_doc = jsonutils.loads(resp[0])
+        self.assertIsInstance(resp_doc, dict)
+        self.assertIn('subscriptions', resp_doc)
+        self.assertIn('links', resp_doc)
+        self.assertEqual([], resp_doc['subscriptions'])
+        self.assertEqual([], resp_doc['links'])
+
     @ddt.data(1, 5, 10, 15)
     def test_listing_works_with_limit(self, limit):
         self._list_subscription(count=15, limit=limit)

zaqar/transport/wsgi/v2_0/subscriptions.py

@@ -142,7 +142,7 @@ class CollectionResource(object):
         kwargs['marker'] = next(results) or kwargs.get('marker', '')
 
         links = []
-        if results:
+        if subscriptions:
             links = [
                 {
                     'rel': 'next',