DBAL-734: [GH-467] [DBAL-472] Fix altering column from notnull to null in Oracle

[doctrinebot]

Jira issue originally created by user @doctrinebot:

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

Url: #467

Message:

Oracle requires an explicit NULL clause when altering a column from NOT NULL to NULL. Otherwise the existing notnull constraint remains unchanged.
This PR changes Oracles column declaration SQL snippet to always specify NULL and NOT NULL in CREATE TABLE and ALTER TABLE statements. This is a perfectly valid syntax and also functional tested.

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-467] was closed:
#467

[doctrinebot]

Issue was closed with resolution "Fixed"

[github-actions]

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