org-element-normalize-contents: Fix when ELEMENT is anonymous

* lisp/org-element.el (org-element-normalize-contents): Use
`org-element-set-contents' to set contents instead of assuming
(TYPE PROP CONTENTS) structure.

lisp/org-element.el

@@ -5331,8 +5331,9 @@ indentation removed from its contents."
 		(lambda (datum)
 		  ;; Return DATUM with all its strings indentation
 		  ;; shortened from MIN-IND white spaces.
-		  (setcdr
-		   (cdr datum)
+                  (apply
+                   #'org-element-set-contents
+                   datum
 		   (mapcar
 		    (lambda (object)
 		      (cond