The update_assignment query clears unset fields

[mabagoury]

In the update assignment query, if some optional fields are not set, EdgeDB clears them (set to Null). But changing these values isn't correct. To avoid losing them, those fields are passed prefilled with the old values from the frontend and FastAPI enforces this through the Assignment Pydantic model.
Some solution should be found at the level of EdgeDB itself. This would provide more flexibility to the frontend only passing the new values.