Handle missing master price when adding a variant

[minhluuquang]

Summary

Fixes #4831.

This prevents the admin variant form from raising when a product's master variant has no default price. The new variant action still copies the master default price when it exists, but skips that clone when it is missing so the rendered form can build a blank default-currency price field.

Testing

• bundle exec rspec spec/controllers/spree/admin/variants_controller_spec.rb
• bundle exec standardrb backend/app/controllers/spree/admin/variants_controller.rb backend/spec/controllers/spree/admin/variants_controller_spec.rb

I ran these in the backend gem context with Ruby 4.0.1 via mise. Full bin/build was not run locally.

[jarednorman]

Is this necessary?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.47%. Comparing base (8d781ac) to head (5ccf596).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6506      +/-   ##
==========================================
- Coverage   89.68%   89.47%   -0.22%     
==========================================
  Files         993      922      -71     
  Lines       20863    19048    -1815     
==========================================
- Hits        18712    17044    -1668     
+ Misses       2151     2004     -147     

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