core: stdcm: avoid a NaN in allowance ratio

[eckter]

Fix a bug where we would fail to find a path for no apparent reason.

Add a few NaN checking assert, as well as some basic logging (in a separate commit)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.49%. Comparing base (d5610e2) to head (0fc0821).
Report is 7 commits behind head on dev.

❗ Current head 0fc0821 differs from pull request most recent head 9ccab99

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Additional details and impacted files

@@              Coverage Diff              @@
##                dev    #7640       +/-   ##
=============================================
+ Coverage     28.47%   87.49%   +59.01%     
=============================================
  Files          1242       31     -1211     
  Lines        153171     1535   -151636     
  Branches       3004        0     -3004     
=============================================
- Hits          43614     1343    -42271     
+ Misses       107769      192   -107577     
+ Partials       1788        0     -1788     

Flag	Coverage Δ
core	?
editoast	?
front	?
gateway	?
railjson_generator	87.49% <ø> (ø)
tests	?

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