DBAL-1028: [GH-709] [DBAL-1028] Fix fetching NULL values via fetchColumn()

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of deeky666:

Url: #709

Message:

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.

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-709] was assigned:
#709

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-709] was closed:
#709

[doctrinebot]

Issue was closed with resolution "Fixed"

[github-actions]

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