Gracefully skip isolation level if no row returned

Gracefully skip isolation level if no row returned

Fixed issue in MySQL dialect when connecting to a psuedo-MySQL database
such as that provided by ProxySQL, the up front check for isolation level
when it returns no row will not prevent the dialect from continuing to
connect. A warning is emitted that the isolation level could not be
detected.

Fixes: #5239
Change-Id: I4a240386a0d38bd90733819495ce50e37fe2234c