use authorId if not empty and not found

footer_with_songinfo.ly

@@ -10,7 +10,9 @@
         (assoc-ref author "death_year")
         (assoc-ref author "organization")
       )
-      "unbekannt")))
+      (if (string-null? authorId)
+          "unbekannt"
+          authorId))))
 
 #(define (find-author-ids-by contributionType authors)
   (if authors