This commit adds `allow_empty_name: true` to the following
endpoints to receive empty strings as topic name when such
topics are available:
* `GET /messages`
* `GET /messages/{message_id}`
* `GET /messages/{message_id}/history`
Also, it adds `empty_topic_name` client capability for spectators.
These changes are part of the broader effort to enable
support for empty string as a topic name.
