src/Pure/Admin/build_release.scala

     Isabelle_System.mkdirs(base_dir)
 
     val release_info =
     {
       val date = Date.now()
-      val name = if (release_name != "") release_name else "Isabelle_" + Date.Format.date(date)
+      val name = proper_string_default(release_name, "Isabelle_" + Date.Format.date(date))
       val dist_dir = base_dir + Path.explode("dist-" + name)
       val dist_archive = dist_dir + Path.explode(name + ".tar.gz")
       val dist_library_archive = dist_dir + Path.explode(name + "_library.tar.gz")
       Release_Info(date, name, dist_dir, dist_archive, dist_library_archive, "")
     }