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/{message_id}/history` endpoint to decide whether
the topic names in the fetched messages can be empty strings.
If False, the topic names in the fetched response will have the
value of `realm_empty_topic_display_name` field in `POST /register`
response replacing "" for channel messages.
293db85f67