front: GET is broken sometimes

[clarani]

What happened?

In this PR, I had to check that the data given to the GET was not empty, otherwise the application would crash (projectedData?.spaceTimeData.length > 0, see @emersion's comment here).

Actually, the GET should not crash if we give empty data to it. We need to investigate what happens here. To reproduce, you can simply remove the condition projectedData?.spaceTimeData.length > 0 in StdcmDebugResults and try to open a space time chart in the stdcm page

What did you expect to happen?

The GET should not make the application crash

How can we reproduce it (as minimally and precisely as possible)?

1. Remove the condition in projectedData?.spaceTimeData.length > 0 in StdcmDebugResults
2. Launch the application
3. Open the stdcm form view
4. Use debug mode
5. Request a stdcm in a timetable with trains (you can create it in the Eex, and then select it with the debug mode)
6. See the GET
7. If it does not crash, try to launch a second request

On which environments the bug occurs?

Local

On which browser the bug occurs?

Firefox

OSRD version (top right corner Account button > Informations)

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