core: prevents a 0-length part in envelope overlay edge case

[eckter]

Fixes #3189

[codecov]

Codecov Report

Merging #3190 (06897ac) into dev (3dbd563) will decrease coverage by 0.04%.
The diff coverage is 14.28%.

@@             Coverage Diff              @@
##                dev    #3190      +/-   ##
============================================
- Coverage     40.36%   40.33%   -0.04%     
  Complexity     1711     1711              
============================================
  Files           630      630              
  Lines         18752    18766      +14     
  Branches       2342     2352      +10     
============================================
  Hits           7570     7570              
- Misses        10780    10782       +2     
- Partials        402      414      +12     

Flag	Coverage Δ
core	83.76% <14.28%> (-0.20%)	⬇️

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Impacted Files	Coverage Δ
.../fr/sncf/osrd/envelope/OverlayEnvelopeBuilder.java	90.69% <0.00%> (-4.43%)	⬇️
.../java/fr/sncf/osrd/envelope/part/EnvelopePart.java	80.08% <16.66%> (-3.40%)	⬇️
...d/infra_state/implementation/TrainPathBuilder.java	81.94% <0.00%> (-1.39%)	⬇️

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