fix get_instance_users_summary IndexError

[QSummerY]

File "/opt/archery/sql/engines/mysql.py", line 401, in get_instance_users_summary
    "is_locked": db_user[3] if server_version >= (5, 7, 6) else None,
IndexError: tuple index out of range

mariadb < 10.4.2 时出现报错

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.30%. Comparing base (ece8c33) to head (f06ea5c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2736      +/-   ##
==========================================
+ Coverage   77.22%   77.30%   +0.08%     
==========================================
  Files         119      119              
  Lines       16296    16316      +20     
==========================================
+ Hits        12584    12613      +29     
+ Misses       3712     3703       -9     

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[LeoQuote]

版本里能否区分mariadb 和mysql 分开判断，其他的变种当成 mysql判断