work if authors not specified

footer_with_songinfo.ly

@@ -13,14 +13,18 @@
       "unbekannt")))
 
 #(define (find-author-ids-by contributionType authors)
-  (filter-map (lambda (authordata) (if (member contributionType (cdr authordata)) (car authordata) #f)) authors))
+  (if authors
+    (filter-map (lambda (authordata) (if (member contributionType (cdr authordata)) (car authordata) #f)) authors)
+    (list)))
 
 #(define (find-author-id-with-part-numbers contributionType authors)
+  (if authors
     (filter-map (lambda (authordata)
       (let ((contributionNumbers (filter-map (lambda (contribution) (if (and (list? contribution) (equal? contributionType (car contribution))) (cadr contribution) #f)) (cdr authordata)))
             (authorId (car authordata)))
         (if (null? contributionNumbers) #f (cons authorId contributionNumbers))
-    )) authors))
+      )) authors)
+    (list)))
 
 #(define-markup-command (print-songinfo layout props) ()
   (define (songinfo-from songId key)