Actually
We can save permissions on artifact field, even when no ugroup
are set.
If "Restrict access" is checked and no ugroups are selected, or if checkbox is uncheked
the artifacts keeps its original permissions.
When no groups are selected it leads to warnings and mysql error.

Expected
When "Restrict access" is checked, we want to select at least one ugroup even
if field is not mandatory.
When required is set on this field we don't want to be able to uncheck the checkbox