fix: remove last 0 byte for mysql_old_password when password is empty

[rainingmaster]

Description

When I create a user with mysql_old_password as auth plugin but without password, and use this user to login(refer this test: https://github.com/rainingmaster/go-mysql-test), it will failed, as my test here, and report Error 1043: Bad handshake.

After my investigation, I have compared the login data from mysql client(which is successful), I found client do not need to send the last 0' byte for the auth switch, if we append last 0` byte even empty password, the hand shake will be broken.

For maradb:5.5 to maradb:10.3, it wont response auth switch by response ok straightly.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[rainingmaster]

@methane @julienschmidt Hi both, could you help me review this PR? So thx

[julienschmidt]

Hi @rainingmaster,
thank you for this PR. In general it looks good to me.

However, kindly add your details to the AUTHORS file, or indicate which existing entry applies.

[rainingmaster]

@julienschmidt Hi

thanks for your review, and I have updated the AUTHORS

[julienschmidt]

LGTM

Thanks!