Fix unit test.

Author: WaVEV

django_mongodb_backend/query_utils.py

@@ -76,6 +76,8 @@ def is_constant_value(value):
     # This should be remove when the same thing above.
     if hasattr(value, "get_source_expressions"):
         simple_sub_expressions = all(map(is_constant_value, value.get_source_expressions()))
+    else:
+        simple_sub_expressions = True
     return (
         simple_sub_expressions
         and isinstance(value, Value)