[653] Handle prefer not to say in ethnic background form

[avinhurry]

Context

This fixes a bug where the prefer not to say radio is not correctly pre-selected.

Changes proposed in this pull request

• Ensure that "Prefer not to say" is correctly stored as ethnic_background with other_background set to nil.

Before/After Video

ethnic_background_prefer_not_to_say_bug.mp4

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card

[avinhurry]

Review all "Check answers" pages