Fix dual table query when system schema is selected database

[harshit-gangal]

Description

If a system schema is selected as a database the dual query fails

use `mysql`;
select @@max_allowed_packet from dual;
ERROR: "keyspace mysql not found in vschema"

The fix is to send to any keyspace if default is system schema.

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving