Users' email, username, password can be updated to empty values

[dpakach]

Actual behavior

When we update an users username, email or password to empty value, it is successful

❯ curl -XPUT http://localhost:9110/ocs/v2.php/cloud/users/hello\?format\=json -d key="email" -d value="" | jq          
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    83  100    67  100    16    632    150 --:--:-- --:--:-- --:--:--   775
{
  "meta": {
    "status": "ok",
    "statuscode": 200,
    "message": "OK"
  },
  "data": {}
}

expected behavior

It should not be possible to update such fields to empty values and it should give a proper error message.