Allow empty list of tags instead of None

If a stack has no tags we want to be able to return an empty list []
instead of None.

Change-Id: I2f31e6b1f59149370e57e5cb43984476e4a1902c
Task: 37005
diff --git a/heat_tempest_plugin/tests/functional/test_stack_tags.py b/heat_tempest_plugin/tests/functional/test_stack_tags.py
index 55c262d..350d2c0 100644
--- a/heat_tempest_plugin/tests/functional/test_stack_tags.py
+++ b/heat_tempest_plugin/tests/functional/test_stack_tags.py
@@ -67,7 +67,7 @@
 
         # Ensure property tag is not populated
         empty_tags_stack = self.client.stacks.get(stack_identifier)
-        self.assertIsNone(empty_tags_stack.tags)
+        self.assertFalse(empty_tags_stack.tags)
 
     @decorators.idempotent_id('5ed79584-0684-4f9c-ae8e-44a8f874ec79')
     def test_hidden_stack(self):