Allow null values for integer columns in declarative schema

[JeroenVanLeusden]

Description (*)

This PR makes it possible to use null as a default value for integer columns.

Fixed Issues (if relevant)

None I could find.

Manual testing scenarios (*)

1. Create db_schema + whitelist in custom module.

<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">
    <table name="cms_page" resource="default">
        <column xsi:type="smallint" name="is_active" nullable="true" default="null"/>
    </table>
</schema>

{
    "cms_page": {
        "column": {
            "is_active": true
        }
    }
}

2. Run bin/magento setup:upgrade
3. is_active column should have a default value null.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

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[sidolov]

@magento run all tests

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-5646 has been created to process this Pull Request
✳️ @sidolov, could you please add one of the following labels to the Pull Request?

Label	Description
Auto-Tests: Covered	All changes in Pull Request is covered by auto-tests
Auto-Tests: Not Covered	Changes in Pull Request requires coverage by auto-tests
Auto-Tests: Not Required	Changes in Pull Request does not require coverage by auto-tests

[engcom-Delta]

✔️ QA passed

[engcom-Foxtrot]

@magento run all tests

[m2-assistant]

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