src/Pure/Admin/build_release.scala

           Isabelle_System.new_directory(context.dist_dir)
           use_release_archive(context, archive, id = rev)
           context
         }
 
-      build_release(options, context, afp_rev = afp_rev,
-        platform_families =
-          if (platform_families.isEmpty) default_platform_families
-          else platform_families,
+      build_release(options, context, afp_rev = afp_rev, platform_families = platform_families,
         more_components = more_components, build_sessions = build_sessions,
         build_library = build_library, parallel_jobs = parallel_jobs, website = website)
     }
   }
 }