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 /users/me/{stream_id}/topics` endpoint to decide
whether the topic names in the fetched `topics` array
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 "".
Fixes part of #23291.
181572021d