SET changes @@default_master_connection if the explicit connection name is blank

# Setup

CHANGE MASTER 'name' TO master_host='127.0.0.1', master_user='root';

SET @@SESSION.default_master_connection= 'name';

SET @@GLOBAL.`name`.replicate_do_db= 'named';

SET @@GLOBAL.``.replicate_do_db= 'unnamed';

SELECT @@GLOBAL.name.replicate_do_db; #=> ❌ unnamed

SELECT @@GLOBAL.``.replicate_do_db; #=> ❌ (empty)

# Cleanup

RESET SLAVE ALL;

SET @@SESSION.default_master_connection= '';

SET @@GLOBAL.replicate_do_db= DEFAULT;