fix: make constraint field optional

[nitishfy]

Closes #3606

warehouse produces the frieght with latest image tag using the following image subscription.

subscriptions:
  - image:
      repoURL: public.ecr.aws/nginx/nginx
      discoveryLimit: 5
      strictSemvers: false
      imageSelectionStrategy: SemVer

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.90%. Comparing base (1d370e3) to head (1b169ee).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5281   +/-   ##
=======================================
  Coverage   55.90%   55.90%           
=======================================
  Files         405      405           
  Lines       29786    29786           
=======================================
  Hits        16653    16653           
  Misses      12168    12168           
  Partials      965      965           

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