feat: code improvement

https://youtrack.weseek.co.jp/issue/GW-7598
- Set slackId as unique in user model
- Implement null check for growiUser variable

packages/app/src/server/models/user.js

@@ -47,7 +47,7 @@ module.exports = function(crowi) {
     name: { type: String },
     username: { type: String, required: true, unique: true },
     email: { type: String, unique: true, sparse: true },
-    slackId: { type: String },
+    slackId: { type: String, unique:true },
     // === Crowi settings
     // username: { type: String, index: true },
     // email: { type: String, required: true, index: true },

packages/app/src/server/service/slack-command-handler/keep.js

@@ -160,7 +160,7 @@ module.exports = (crowi) => {
 
     messages.map(async(message) => {
       const growiUser = growiUsers.find(user => user.slackId === message.user);
-      if (growiUser) {
+      if (growiUser !== null) {
         message.user = `${growiUser.name} (@${growiUser.username})`;
       }
       else {