Segmentation fault when Kirby task is aborted at start

[martinburchell]

Seen on Qt6.5 branch but could also a problem with Qt5.12.

Dereferencing the output of max_element() for an empty list gives undefined behaviour.

https://github.com/ucam-department-of-psychiatry/camcops/blob/0c2d3465d5ccc6b28bcd290ba877061782142458/tablet_qt/tasks/kirby.cpp#L368