[Tableau] - default db name cannot be an empty string

[kuba--]

So far a default database name in an empty string. Tableau requires non-empty db names to populate UI. We should pick some non-empty, default db name and implement support for that.

[kuba--]

Moreover we should extra support following related queries:

DROP TABLE `#Tableau_01_sid_00000001_1_Connect`
SELECT CONNECTION_ID()
show variables like 'collation_server'
SELECT `table_name`, `column_name`\nFROM `information_schema`.`columns`\nWHERE `data_type`='enum' AND `table_schema`=''
SHOW DATABASES LIKE '%'