Fix to do not show empty parentheses if no Author's role.

lib/decentralised_book_index_web/live/book_live/show.ex

@@ -121,7 +121,7 @@ defmodule DecentralisedBookIndexWeb.BookLive.Show do
     authors_string =
       author_roles
       |> Enum.map(fn author_role ->
-        if author_role.role == "" do
+        if author_role.role in ["", nil] do
           {"#{author_role.author.name}", author_role.author.id}
         else
           {"#{author_role.author.name} (#{author_role.role})", author_role.author.id}