Better test coverage for Material Item with null Material List

further improvements after #7217

CreateActivityTest:
- [x] copy an activity from another camp -> materiallist is null
- [ ] copy an activity from another camp, and then copy it again in the same camp
- [ ] copy an activity from another camp, and then copy it to a third camp

The following test would be nice:
update a materialitem with a null material list

- [ ] if you set the materiallist to not null -> success
- [ ] if you change another property and leave the materiallist null -> error
- [ ] if you change the material list from null to a material list of another camp -> error

GUI:
- [x] Material - show a dummy-list for 'unasigned material' #8053