Material-Item . MaterialList nullable - further improvements

[pmattmann]

further improvements after #7217

CreateActivityTest:

• 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:
Materail - show a dummy-list for 'unasigned material'

[simfeld]

How do we move on here after the revert in #7236 due to bad query performance of the change? @pmattmann I assume you have some ideas already?