Dashboard consumer undefined user

[Applenice]

Issue description

Undefined users can be created in Consumer List via Dashboard

Expected behavior

The appearance of undefined users does not make sense

How to Reproduce

Create two Comsumer and enable plugin configuration
First，the configuration is as follows:

{
  "username": "Pokemon",
  "plugins": {
    "basic-auth": {
      "disable": false,
      "password": "Pokemon",
      "username": "Pokemon"
    },
    "ip-restriction": {
      "disable": false,
      "whitelist": [
        "192.168.0.112"
      ]
    }
  }
}

In fact, you can create undefined users in this step:
Consumer List->First Consumer Pokemon-> Operation View-> click Submit，you can see that there is an additional undefined user in the list, except for the username is different, other configuration and Pokemon consistent.

{
  "username": "undefined",
  "plugins": {
    "basic-auth": {
      "disable": false,
      "password": "Pokemon",
      "username": "Pokemon"
    },
    "ip-restriction": {
      "disable": false,
      "whitelist": [
        "192.168.0.112"
      ]
    }
  }
}

Second，the configuration is as follows:

{
  "username": "Pikachu",
  "plugins": {
    "basic-auth": {
      "disable": false,
      "password": "Pikachu",
      "username": "Pikachu"
    },
    "ip-restriction": {
      "disable": false,
      "whitelist": [
        "127.0.0.1"
      ]
    }
  }
}

Next, the operation:
Consumer List->Second Consumer Pikachu-> Operation View-> click Submit，you can see that the configuration of the undefined user has been modified, except for the username, which is the same as Pikachu, so I won't post it

Screenshots

Environment

• apisix version (cmd: apisix version): 2.6
• OS (cmd: uname -a):Ubuntu 1804
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): openresty/1.19.3.2
• etcd version, if have (cmd: run curl http://127.0.0.1:9090/v1/server_info to get the info from server-info API): 3.4.13
• apisix-dashboard version, if have: release/2.7
• Browser version, if have: Firfox 89.0.1

[juzhiyuan]

cc @iamayushdas to take a look

[iamayushdas]

okay

[iamayushdas]

yes @Applenice there is a bug confirmed