[Trainings] Adding a session-specific price is broken

[maieutiquer]

Q	A
Bug report?	yes
Version?	13.2 - 13.5

Steps:

1. Install 13.2 or 13.5 from scratch
2. Activate Tarification
3. Add a training with price and a session
4. Try modifying the session's price and apply

Expected: The price should be saved

Actual: The price is not saved and there's no error message