Omit empty FormResponse errors from analytics logging

[aduth]

🛠 Summary of changes

Updates FormResponse#to_h to serialize an empty errors as nil instead of {}, effectively removing this value from logging.

This serves two purposes:

• Reduce the size (and cost) of our logging payloads by removing values which aren't relevant for querying
• Iterative step toward removing errors altogether.
  • These changes simplify future efforts by proactively removing assertions of errors: {}

📜 Testing Plan

Verify build passes.

Verify using make watch_events that any event which would have previously logged errors: {} will now exclude errors from the logging payload.