clean code

src/server/routes/apiv3/slack-bot.js

@@ -10,6 +10,7 @@ module.exports = (crowi) => {
   const requestHandler = boltService.receiver.requestHandler.bind(boltService.receiver);
 
   router.post('/', async(req, res) => {
+    // for verification request URL on Event Subscriptions
     if (req.body.type === 'url_verification') {
       res.send(req.body);
       return;

src/server/service/bolt.js

@@ -37,7 +37,7 @@ class BoltReciever {
           throw new Error(message);
         }
         else if (!response) {
-          return;
+          return null;
         }
         else {
           return response;