Empty use restriction form is validated

[vildead]

Describe the bug (current behaviour)
Empty use restriction in data declaration edit form is validated - it must be first deleted in order to Submit successfully.

To reproduce

1. Go to Data declaration edit page
2. fill in all required fields but leave the last (extra) restriction form empty
3. submit

this should successfully pass since the restriction form is empty - but it is failing with "This field is required" on Use category field (required restriction_class field).

Expected behaviour
Since it is an 'extra' form, the validation should be skipped if the data did not change (at least according to the doc). But it seems the form.has_changed attribute is True in the view.

Environment:
bd88a9e