Wrong using password flag when the user is not found

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql -h127.1 -udummy -P6000 -pdummy
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'dummy'@'127.0.0.1' (using password: NO)

The user dummy is not created.

2. What did you expect to see? (Required)

ERROR 1045 (28000): Access denied for user 'dummy'@'127.0.0.1' (using password: YES)
The value of using password should be YES.

3. What did you see instead (Required)

ERROR 1045 (28000): Access denied for user 'dummy'@'127.0.0.1' (using password: NO)
The value of using password is NO.

4. What is your version? (Required)

master

[djshow832]

The reason:
When TiProxy handshakes with TiDB, it sends auth_plugin=unknown_auth_plugin and auth_data=nil. TiDB sees that auth_data=nil and then sets using password to NO.