fix mock error caused by invalid username

src/mock/users.js

@@ -61,7 +61,7 @@ module.exports = {
     const { username, password } = req.body
     const user = adminUsers.filter((item) => item.username === username)
 
-    if (user && user[0].password === password) {
+    if (user[0] && user[0].password === password) {
       const now = new Date()
       now.setDate(now.getDate() + 1)
       res.cookie('token', JSON.stringify({ id: user[0].id, deadline: now.getTime() }), {