Handle 'notspecificcountry' country_code value

[baarkerlounger]

Description of change

Resolve: https://sentry.ci.uktrade.digital/organizations/dit/issues/139199/?project=235&query=is%3Aunresolved

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (4bf8182) to head (75a8d0c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5805      +/-   ##
==========================================
- Coverage   96.48%   96.47%   -0.01%     
==========================================
  Files        1037     1037              
  Lines       24621    24621              
  Branches     1637     1637              
==========================================
- Hits        23755    23754       -1     
- Misses        709      710       +1     
  Partials      157      157              

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