fix: num sig zero is invalid

[thesimplekid]

Description

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Address nut change cashubtc/nuts#358

Notes to the reviewers

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Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• I followed the code style guidelines
• I ran just quick-check before committing
• If the Wallet API was modified (added/removed/changed), I have reflected those changes in the FFI bindings (crates/cdk-ffi)

[codecov]

Codecov Report

❌ Patch coverage is 98.46743% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.32%. Comparing base (e06f337) to head (e99a665).

Files with missing lines	Patch %	Lines
crates/cashu/src/nuts/nut10/spending_conditions.rs	93.54%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1966      +/-   ##
==========================================
+ Coverage   65.16%   65.32%   +0.15%     
==========================================
  Files         330      330              
  Lines       56742    56997     +255     
==========================================
+ Hits        36976    37232     +256     
+ Misses      19766    19765       -1     

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