Fixed #19173 -- Made EmptyQuerySet a marker class only

The guarantee that no queries will be made when accessing results is
done by new EmptyWhere class which is used for query.where and having.

Thanks to Simon Charette for reviewing and valuable suggestions.

tests/modeltests/basic/tests.py

@@ -4,6 +4,7 @@ from datetime import datetime
 
 from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
 from django.db.models.fields import Field, FieldDoesNotExist
+from django.db.models.query import EmptyQuerySet
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
 from django.utils.translation import ugettext_lazy
@@ -639,3 +640,9 @@ class ModelTest(TestCase):
         Article.objects.bulk_create([Article(headline=lazy, pub_date=datetime.now())])
         article = Article.objects.get()
         self.assertEqual(article.headline, notlazy)
+
+    def test_emptyqs(self):
+        # Can't be instantiated
+        with self.assertRaises(TypeError):
+            EmptyQuerySet()
+        self.assertTrue(isinstance(Article.objects.none(), EmptyQuerySet))