schemas: provide a default value for ETCS brake parameters

[woshilapin]

Without this default value, it's a breaking change and consumers of this repository will have to update their use of RollingStock class.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.63%. Comparing base (673dc44) to head (7da888c).
Report is 24 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10340      +/-   ##
==========================================
- Coverage   81.64%   81.63%   -0.02%     
==========================================
  Files        1066     1066              
  Lines      105687   105687              
  Branches      727      727              
==========================================
- Hits        86291    86280      -11     
- Misses      19355    19366      +11     
  Partials       41       41              

Flag	Coverage Δ
editoast	73.64% <ø> (-0.04%)	⬇️
front	89.33% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.50% <ø> (ø)
tests	87.05% <ø> (ø)

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[bougue-pe]

🙏 Thanks!