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

`PDO` drivers return `null` when retrieving a SQL `NULL` value from a column via `Statement::fetchColumn()`.
However all native driver implementations except `mysqli` return `false` which according to the interface should only be returned if no more rows are available or an error occurred. So currently it is not possible to retrieve a SQL `NULL` value via `Statement::fetchColumn()` with those drivers.
This PR fixes this. Additionally if a non-existing column index is requested from the resultset, `PDO` also returns `null` whereas those mentioned native drivers return `false`. This is also fixed.
You can actually skip `array_key_exists()` here, as the return value will be `null` anyway