Return empty list instead of None for tags

If a stack has no tags, return an empty list [] instead of None for its
tags list.

Change-Id: I3c3c083d7261172d08b933de7c63b6a489ed6ccd
Depends-On: https://review.opendev.org/687769
Task: 37009

heat/engine/api.py

@@ -221,7 +221,7 @@ def format_stack(stack, preview=False, resolve_outputs=True):
         rpc_api.STACK_OWNER: stack.username,
         rpc_api.STACK_PARENT: stack.owner_id,
         rpc_api.STACK_USER_PROJECT_ID: stack.stack_user_project_id,
-        rpc_api.STACK_TAGS: stack.tags or None,
+        rpc_api.STACK_TAGS: stack.tags,
     }
 
     if not preview:

heat/tests/test_engine_api_utils.py

@@ -398,7 +398,7 @@ class FormatTest(common.HeatTestCase):
             'outputs': [],
             'template_description': 'No description',
             'timeout_mins': None,
-            'tags': None,
+            'tags': [],
             'parameters': {
                 'AWS::Region': 'ap-southeast-1',
                 'AWS::StackId': aws_id,