data-manager: Account for cardinality=0 on DB migration

If a property changes from maxCardinality 1 to many, the database
format is updated to cope with that, but at the time of migrating
data, it doesn't account for resources having no elements. In order
to avoid constraint errors, those must be skipped.

https://bugzilla.gnome.org/show_bug.cgi?id=743727