Fix FixedCurrencyMoneyInput.decompress to always return currency

[filipmlynarski-mirumee]

Fix: Return currency in FixedCurrencyMoneyInput.decompress() when value is None

Problem

FixedCurrencyMoneyInput.decompress() returns [None, None] when value is None, causing the hidden currency input to have no value. This leads to form validation errors and data corruption.

Solution

Changed to return [None, self.currency] to ensure the currency is always available.

Impact

• Fixes form submission failures with empty price fields
• Prevents database corruption from empty currency codes
• Backwards compatible - only affects empty/None values

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.94%. Comparing base (eb5dfe5) to head (16174a6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   97.70%   97.94%   +0.24%     
==========================================
  Files          13       13              
  Lines         479      487       +8     
==========================================
+ Hits          468      477       +9     
+ Misses         11       10       -1     

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