Fix set_time of ChoreStartTime class

[samuelko123]

It attempts to fix 2 issues:

1. We cannot set minute to 0, because "if minute:" will evaluate to False. The same goes for other parameters.
2. We would get "ValueError: day is out of range for month" if we set month to 2 while the original day is 31, vice versa.

[MariusWirtz]

Thanks for the fix!