Empty reservable from causes errors in both frontend and backend

[EwoutV]

When updating a timeslot from not being reservable to being reservable, a backend API call can result in a inconsistent state where the timeslot doesn't have a valid reservable from date. Currently, the backend code can't cope with this scenario and crashes with a null pointer exception.

[EwoutV]

The issue originates from the frontend, where updating a firstly not reservable timeslot to being reservable throws a null pointer exception that makes it impossible to configure the reservable from. A subsequent click on the update button calls the backend and stores the now reservable timeslot without a reservable from date.