Handle missing contacts in contact update action serialization

[maricavor]

Summary

• Fix NoMethodError (undefined method 'code' for nil:NilClass) in Action#serialized_contact when serializing contact update notifications
• Wavecom (and potentially other registrars) could not access the registrar portal — /repp/v1/registrar/summary returned 500, causing a redirect loop (too_many_redirects) in the frontend
• Root cause: an unread ContactUpdateAction notification where the action or one of its bulk subactions had a nil contact (likely from a registrant auto-update where contact save failed or contact was removed)
• Action#to_non_available_contact_codes now skips nil contacts instead of crashing; affected endpoints return an empty contacts list for the broken entries