tests to come

Disallow empty delete criteria on the connection

MasterSlaveConnection::close() should reset connections array

Fixed undefined index notice when attempting to reconnect a MasterSlaveConnection after close has called

Close #722

Add @return type hint

Close #714

#714 - ignoring PDOExceptions on PDO#getAttribute(PDO::PGSQL_ATTR_DISABLE_PREPARES) as per https://bugs.php.net/bug.php?id=68351

#714 - Adding a test verifying that attribute `PDO::PGSQL_ATTR_DISABLE_PREPARES` is set when using pgsql on PHP 5.6+

Fix tear down foreign key constraint violation exception tests

[DBAL-1024] Add more foreign key constraint violation error codes

[DBAL-1028] Fix fetching NULL values via fetchColumn()