core: avoid adding an invalid cache entry when using an unloaded infra

[eckter]

Fixes a bug that would prevent infra loading when receiving a request before the infra has started loading.

We would put a default InfraCacheEntry (INITIALIZING) when missing from the cache, but the infra isn't actually being loaded.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (39c05f1) 27.70% compared to head (5952343) 27.70%.
Report is 1 commits behind head on dev.

Files	Patch %	Lines
...e/src/main/java/fr/sncf/osrd/api/InfraManager.java	25.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #4496      +/-   ##
============================================
- Coverage     27.70%   27.70%   -0.01%     
  Complexity     2165     2165              
============================================
  Files          1002     1002              
  Lines        126258   126257       -1     
  Branches       2580     2580              
============================================
- Hits          34979    34976       -3     
- Misses        89790    89791       +1     
- Partials       1489     1490       +1     

Flag	Coverage Δ
core	78.58% <25.00%> (-0.01%)	⬇️
editoast	75.74% <ø> (-0.01%)	⬇️
front	8.35% <ø> (ø)
gateway	2.50% <ø> (ø)
railjson_generator	87.42% <ø> (ø)
tests	81.77% <ø> (ø)

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