front: fix unnecessary type casting in upsert path step

[Synar]

Close #10805

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.92%. Comparing base (3af0ded) to head (0ddf291).
Report is 44 commits behind head on dev.

Files with missing lines	Patch %	Lines
front/src/reducers/osrdconf/helpers.ts	66.66%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10825      +/-   ##
==========================================
+ Coverage   81.87%   81.92%   +0.05%     
==========================================
  Files        1083     1084       +1     
  Lines      107157   107214      +57     
  Branches      741      738       -3     
==========================================
+ Hits        87734    87840     +106     
+ Misses      19382    19332      -50     
- Partials       41       42       +1     

Flag	Coverage Δ
editoast	74.37% <ø> (-0.02%)	⬇️
front	89.49% <66.66%> (+0.09%)	⬆️
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.58% <ø> (ø)
tests	87.91% <ø> (ø)

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[emersion]

Nice!

[Math-R]

lgtm