Fix nil pointer in runner group if not modified

[dbaur]

Resolves #1524

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Behavior

Before the change?

• reading the github_actions_runner_group will lead to a nil pointer exception if the API returns a 304 not modified response

After the change?

• the read method will return early (not setting the attributes on the schema) if the resource is not modified

Other information

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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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[nickfloyd]

Thanks, @dbaur for the contribution here! ❤️