Fix wins dataset endpoint for responded_on and sector.

[elcct]

Description of change

This ensures that when customer_response__responded_on is null, the customer response properties are also null in the dataset.

It also corrects the sector_display so that it returns the entire sector, not just the segment.

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 96.52%. Comparing base (7d04233) to head (a34a5ce).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5331   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files         925      925           
  Lines       21947    21947           
  Branches     1962     1962           
=======================================
+ Hits        21184    21185    +1     
  Misses        617      617           
+ Partials      146      145    -1     

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