Relax boolean fields for Legacy export wins.

[elcct]

Description of change

This relaxes Boolean fields in customer response to accommodate legacy wins without customer response.
This also updates handling of other fields that can be missing in the source data.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (e1580b1) to head (8cbb476).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5406      +/-   ##
==========================================
- Coverage   96.40%   96.39%   -0.01%     
==========================================
  Files         938      939       +1     
  Lines       22328    22332       +4     
  Branches     2002     2003       +1     
==========================================
+ Hits        21526    21528       +2     
- Misses        654      655       +1     
- Partials      148      149       +1     

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