fix: Replace badCurrency() checks with isBadCurrency() for improved clarity

[tequdev]

High Level Overview of Change

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.58%. Comparing base (9b50b68) to head (a272926).

Files with missing lines	Patch %	Lines
src/libxrpl/protocol/Issue.cpp	0.00%	0 Missing and 1 partial ⚠️
src/libxrpl/protocol/STCurrency.cpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #742   +/-   ##
=======================================
  Coverage   66.57%   66.58%           
=======================================
  Files         837      837           
  Lines       78540    78540           
  Branches    44600    44600           
=======================================
+ Hits        52288    52292    +4     
+ Misses      17859    17853    -6     
- Partials     8393     8395    +2     

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