Add undefined_attributes field to product syncing

[carterbuce]

Description

The /fb-checkout URL does not work for product variants that use the "Any Size", "Any Color", etc attribute selection. They must define the attribute. This PR sends a list of the attributes that do not have a defined value to Meta, so that we can hide those variants from Shops but still allow them to be synced over for use in Ads.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• [] I have commented my code, particularly in hard-to-understand areas.
• [] I have confirmed that my changes do not introduce any new PHPCS warnings or errors.
• [] I followed general Pull Request best practices. Meta employees to follow this wiki.
• [] I have added tests (if necessary) and all the new and existing unit tests pass locally with my changes.
• [] I have completed dogfooding and QA testing, or I have conducted thorough due diligence to ensure that it does not break existing functionality.
• [] I have updated or requested update to plugin documentations (if necessary). Meta employees to follow this wiki.

Changelog entry

Add undefined_attributes field to product syncing

Test Plan

Synced a product to Meta and verified the undefined_attributes field is sent

Screenshots

Please provide screenshots or snapshots of the system/state both before and after implementing the changes, if appropriate

Before

After

[facebook-github-bot]

@carterbuce has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@carterbuce has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@carterbuce has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@carterbuce has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.