Improve handling of missing fields in Legacy export wins.

[elcct]

Description of change

Some legacy export wins have fields missing or coming with null value which was not handled properly.
This fixes it.

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 96.40%. Comparing base (5a4df6a) to head (2462763).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5415      +/-   ##
==========================================
- Coverage   96.40%   96.40%   -0.01%     
==========================================
  Files         939      940       +1     
  Lines       22335    22341       +6     
  Branches     2003     2003              
==========================================
+ Hits        21533    21537       +4     
- Misses        655      656       +1     
- Partials      147      148       +1     

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