Make CreatedAt nullable

[lineville]

Resolves #2733 by making the newly added CreatedAt field on the WorkflowJob model nullable.

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Behavior

Before the change?

• Deserializing calls that return a WorkflowJob would fail if running against GHES < 3.9 since the CreatedAt field is not provided on older GHES versions.

After the change?

• Deserializing calls that return a WorkflowJob succeed running against GHES < 3.9 since the CreatedAt field is nullable and not provided by older GHES versions.

Other information

• This is a quick follow-up fix to [FEAT]: Add CreatedAt field to WorkflowJob Model #2728 -- would be great to get this fix in 🙏 @kfcampbell

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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