Query : possible error around conditional removing visitor and sql functions

[maumar]

Problem so far only surfaces in MySql provider, but potentially it's a wider issue that can be reproduced in Sql provider with a correct query.

More info: #6687 (comment)
Mysql repro: http://bugs.mysql.com/bug.php?id=84505

[gmartinezsan]

@maumar This issue has been already addressed in the MySQL provider.