Fix adding related dossiers to an existing dossier.

The following error occurred rendering the ou-selector viewlet while traversing
on to the related-dossier selection widget:
`NullOrgUnit`, i.e. `OrgUnit` null implementation, was missing a method. We
added a test to make sure that the null-implementation always implements the
same "interface" as `OrgUnit`.

 Fixes #324.