Fix false mpp payment handling of unsupported backends

[callebtc]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 17 lines in your changes missing coverage. Please review.

Project coverage is 65.43%. Comparing base (3b2f1aa) to head (8e765ae).
Report is 13 commits behind head on main.

❗ Current head 8e765ae differs from pull request most recent head d2a530c

Please upload reports for the commit d2a530c to get more accurate results.

Files	Patch %	Lines
cashu/mint/ledger.py	63.88%	13 Missing ⚠️
cashu/core/models.py	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
- Coverage   69.62%   65.43%   -4.19%     
==========================================
  Files          76       81       +5     
  Lines        6913     7363     +450     
==========================================
+ Hits         4813     4818       +5     
- Misses       2100     2545     +445     

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[lollerfirst]

This will most likely fix the problem.