user_topic: Add support for empty topic name.

This commit is a part of the work to support empty string
as a topic name.

Previously, empty string was not a valid topic name.

Now, toggling topic visibility policy operation supports
empty topic name.

Adds backward compatibility for:
- `topic_name` field in the `user_topic` event type

api_docs/changelog.md

@@ -41,6 +41,7 @@ format used by the Zulip server that they are interacting with.
     * `subject` field in the `message` event type
     * `topic` field in the `delete_message` event type
     * `orig_subject` and `subject` fields in the `update_message` event type
+    * `topic_name` field in the `user_topic` event type
 
 * [`GET /messages`](/api/get-messages),
   [`GET /messages/{message_id}`](/api/get-message): Added `allow_empty_topic_name`