Avoid empty hash default values in analytics methods

[aduth]

🛠 Summary of changes

Updates AnalyticsEvents method signatures to avoid assigning a default empty hash value {}, preferring nil instead to ensure values are omitted from logs.

As with #11799, this serves to reduce the size (and cost) of our logging payloads by removing values which aren't relevant for querying. It is not possible to query for an empty hash value in CloudWatch Insights.

This is also an iterative step in the migration away from errors to error_details which will involve making error_details a guaranteed return value from FormResponse#to_h.

📜 Testing Plan

Verify build passes.