Fix ethnic background form no method error

[CatalinVoineag]

Context

Sentry error https://dfe-teacher-services.sentry.io/issues/5774209867/events/66d8e6e0c7964544a0e375e3dfd1f540/

We need to rescue times where the ethnic group passed is not present

Changes proposed in this pull request

Guidance to review

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