Fix trailing space for users without a last name

[cschuijt]

On Telegram, users can have their last name not set. Constructing a full name like "#{request.params["first_name"]} #{request.params["last_name"]}" with last_name = nil then gives you the first name with a space at the end. To fix this, I added a private function to your strategy that checks if there's a last name. If yes, it will build the full name the same way. If not, it will just return the first name, without a space at the end.