Don't error out on route53.list_tags_for_resource when resource has no tags

Without the added `return {}`, calling route53.list_tags_for_resource
when called with a ResourceId of a resource without any tags would
result in the error:
    jinja2.exceptions.UndefinedError: 'None' has no attribute 'items'

Because the LIST_TAGS_FOR_RESOURCE_RESPONSE was given None instead of
empty dict.

This now allows list_tags_for_resource to be called without issue on
tag-less resources.

moto/route53/models.py

@@ -305,6 +305,7 @@ class Route53Backend(BaseBackend):
     def list_tags_for_resource(self, resource_id):
         if resource_id in self.resource_tags:
             return self.resource_tags[resource_id]
+        return {}
 
     def get_all_hosted_zones(self):
         return self.zones.values()