Mint: Allow 0-valued amounts for blank outputs

[callebtc]

Compatibility issues with older version of cashu-ts

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (3e58670) 68.01% compared to head (acf0c1b) 67.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #348      +/-   ##
==========================================
- Coverage   68.01%   67.97%   -0.05%     
==========================================
  Files          63       63              
  Lines        4834     4834              
==========================================
- Hits         3288     3286       -2     
- Misses       1546     1548       +2     

Files	Coverage Δ
cashu/mint/ledger.py	79.07% <100.00%> (ø)
cashu/mint/verification.py	79.48% <100.00%> (ø)
cashu/wallet/wallet.py	88.75% <100.00%> (ø)

... and 1 file with indirect coverage changes

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