Fix an error in org-taskjuggler-get-unique-id

Don't try to pop from an empty list and downcase the result

lisp/org-taskjuggler.el

@@ -503,7 +503,7 @@ finally add more underscore characters (\"_\")."
 	 (parts (split-string headline))
 	 (id (org-taskjuggler-clean-id (downcase (pop parts)))))
     ; try to add more parts of the headline to make it unique
-    (while (member id unique-ids)
+    (while (and (member id unique-ids) (car parts))
       (setq id (concat id "_" (org-taskjuggler-clean-id (downcase (pop parts))))))
     ; if its still not unique add "_"
     (while (member id unique-ids)