core: envelope: add check for case where train can't start

[eckter]

Fixes #3628

[codecov]

Codecov Report

Merging #3629 (ef8132e) into dev (de31346) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                dev    #3629   +/-   ##
=========================================
  Coverage     68.37%   68.37%           
- Complexity     1906     1908    +2     
=========================================
  Files           421      421           
  Lines         21221    21225    +4     
  Branches       1540     1542    +2     
=========================================
+ Hits          14509    14513    +4     
  Misses         5994     5994           
  Partials        718      718           

Flag	Coverage Δ
core	79.18% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
...osrd/envelope_sim/pipelines/MaxEffortEnvelope.java	98.24% <100.00%> (+0.13%)	⬆️

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