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.

django/db/models/sql/query.py

@@ -25,7 +25,7 @@ from django.db.models.sql.constants import (QUERY_TERMS, ORDER_DIR, SINGLE,
 from django.db.models.sql.datastructures import EmptyResultSet, Empty, MultiJoin
 from django.db.models.sql.expressions import SQLEvaluator
 from django.db.models.sql.where import (WhereNode, Constraint, EverythingNode,
-    ExtraWhere, AND, OR)
+    ExtraWhere, AND, OR, EmptyWhere)
 from django.core.exceptions import FieldError
 
 __all__ = ['Query', 'RawQuery']
@@ -1511,6 +1511,13 @@ class Query(object):
             self.add_filter(('%s__isnull' % trimmed_prefix, False), negate=True,
                     can_reuse=can_reuse)
 
+    def set_empty(self):
+        self.where = EmptyWhere()
+        self.having = EmptyWhere()
+
+    def is_empty(self):
+        return isinstance(self.where, EmptyWhere) or isinstance(self.having, EmptyWhere)
+
     def set_limits(self, low=None, high=None):
         """
         Adjusts the limits on the rows retrieved. We use low/high to set these,