Return BigDecimal from FlatRate zero branches

[minhluuquang]

Description

Fixes #3756.

Spree::Calculator::FlatRate#compute returned an integer 0 when called without an object, with a blank preferred currency, or with a currency mismatch. Calculator results are expected to be BigDecimal monetary amounts, so this changes those zero branches to return Spree::ZERO.

This also updates the Calculator YARD return type and adds specs covering the FlatRate zero branches.

Testing

• bundle exec rspec spec/models/spree/calculator/flat_rate_spec.rb
• bundle exec rspec spec/models/spree/calculator_spec.rb spec/models/spree/calculator
• git diff --check

Checklist:

• I have followed the Pull Request guidelines.
• I have added a detailed description into the commit message.
• Guides and README updates are not needed for this bug fix.
• I have added tests to cover this change.
• Screenshots are not applicable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (8d781ac) to head (0302ec3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6507   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         993      993           
  Lines       20863    20863           
=======================================
  Hits        18712    18712           
  Misses       2151     2151           

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[tvdeyen]

Thanks. Linting issue is unreleased and already fixed in another PR