hooks: Call the callback when done

If a hook function neither calls the callback nor returns a
(non-undefined) value then there's no way for the hook system to know
if/when the hook function has finished.

src/node/hooks/express/importexport.js

@@ -71,4 +71,6 @@ exports.expressCreateServer = function (hook_name, args, cb) {
     }
     await importHandler.doImport(req, res, req.params.pad);
   });
+
+  return cb();
 }