Fix database checker PDO execute statement

[norkus256]

PDO exec function is not supported for queries that return results.

https://www.php.net/manual/en/pdo.exec.php#61702

It does not cause an error, but notes in MySQL log file

[Note] Aborted connection 3 to db: 'my_database' user: 'root' host: 'localhost' (Got an error reading communication packets)

You can check that aborted clients count is always increasing

SHOW STATUS LIKE '%Aborted_clients%';