fetch_messages: Add support for empty topic names.

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.

Adds `allow_empty_topic_name` boolean parameter to `GET /messages`
and `GET /messages/{message_id}` endpoints to decide whether the
topic names in the fetched messages can be empty strings.

If False, the topic names in the fetched message will have the
value of `realm_empty_topic_display_name` field in `POST /register`
response replacing "" for channel messages.

zerver/tests/test_message_edit.py

@@ -49,6 +49,7 @@ class EditMessageTest(ZulipTestCase):
                 search_fields={},
                 apply_markdown=False,
                 client_gravatar=False,
+                allow_empty_topic_name=True,
                 allow_edit_history=True,
                 user_profile=None,
                 realm=msg.realm,