Negative values for tardiness are not rejected

If a teacher enters a minus number for tardiness, the weekly overview is no longer displayed afterwards.

added type::bug label

added part::backend label

created merge request !146 (merged) to address this issue

mentioned in merge request !146 (merged)

mentioned in commit ab1f4a35

closed with commit ab1f4a35

closed with commit fc121ed3

mentioned in commit fc121ed3