Test switchovers with null batonid's

[redfast00]

At the moment, there are two occasions when we'd like to use a null batonid in a batonswitchover:

• a null previousbatonid: this is useful at the start of the competition, so teams are only assigned the detections of the baton after the competition has started
• a null newbatonid: this is useful at the end of the competition (or when a team stops running)

We used to do the previousbatonid null case by just having a fixed baton that isn't used and using that id. This works, but is ugly in the database. At the current 6urenloop, we used null, which worked for the HMM lapper, but not for RobustLapper (fixed now)

We still use a fixed newbatonid of a baton that is not in use for the end of the competition; we should test that a null id works for newbatonid to deassign a baton