Fix nested checkboxes with single top-level node
Fix nested checkboxes with single top-level node

Nested checkboxes with a single top-level node
will only have one item in their `items` list.
This is because the other choices are children of
that list item.

This means we need to check the `choices`
attribute, which lists all the checkboxes, to see
if they should be marked as a group (by being
wrapped in a `<fieldset>`) or not.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin