front: editor: slopes and curves can't be empty

[eckter]

What happened?

When creating a track, the slopes and curves can't be empty. This is especially weird for curves, as the semantically correct way to have straight tracks (other that not having value) is to have an infinite radius. Inputting 0 works as well but that's mostly by chance (core avoiding a division by 0)

What did you expect to happen?

Slopes and curves should accept empty lists

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

Edit a track and try not to input curves

What operating system, browser and environment are you using?

• Browser: Firefox
• OS: Ubuntu
• Env: dev

OSRD version (top right corner Account button > Informations)

c39e699

[clarani]

Solved by #5581, duplicated with #3479