Fix error on atom feed with no authors

lib/parser/atom.ex

@@ -72,7 +72,13 @@ defmodule FeedParser.Parser.Atom do
             _ -> nil
           end
 
-        author = texts('/entry/author/name/text()', entry) || feed_author
+        author =
+          texts('/entry/author/name/text()', entry) ||
+            feed_author
+            |> case do
+              nil -> nil
+              authors -> Enum.join(authors, ", ")
+            end
 
         content = text('/entry/content/text()', entry) || text('/entry/summary/text()', entry)
 
@@ -83,7 +89,7 @@ defmodule FeedParser.Parser.Atom do
           links: links,
           content: content,
           date: updated,
-          creator: author |> Enum.join(", ")
+          creator: author
         }
       end)