Yes thats correct, you are supposed to
If display name is set and not blank use display name else use username/login
I checked on the API here https://api.twitch.tv/kraken/users/gustavoniro theres no “special” Display Name beyond the “default” of “uppercase the first character”
As per the docs I linked earlier:
display-name is the user's display name, escaped as described in the IRCv3 spec.
Empty if it's never been set.