Make order field of sets required

[rolandgeider]

Steps to Reproduce

The order field for the sets needs to be required, otherwise an exception will be thrown when sorting. We also need to check how/if the backend enforces this

Logs

When the exception was thrown, this was the stack: 
#0      _WorkoutDayWidgetState.initState.<anonymous closure> (package:wger/widgets/workouts/day.dart:104:33)
#1      Sort._insertionSort (dart:_internal/sort.dart:69:36)
#2      Sort._doSort (dart:_internal/sort.dart:58:7)
#3      Sort.sort (dart:_internal/sort.dart:33:5)
#4      ListMixin.sort (dart:collection/list.dart:356:10)