Use safe dict access for yanked fields

[jobselko]

Follow-up to #1282

This is needed to avoid a KeyError at runtime for pulp-service, whose patch 0060 passes release dicts to write_simple_detail_json without yanked and yanked_reason keys.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough