Refs. #3522. The core problem is the same as for groups/features: we can't
allow empty trusts when they're unmarshalled from JSON. But unlike others we
can't easily differentiate missing any value with other cases because the
default value for WildPermissionDescs is a valid thing. Adding an additional
field makes it invalid and we can build around it. Other options are
implementing custom UnmarshalJSON for Manifest (too much for this) or making
Trusts a pointer (an option, but can fail in too many ways).
Signed-off-by: Roman Khimov <roman@nspcc.ru>
