"Date unknown" is inconsistent with our handling of unknown values elsewhere

See http://test.musicbrainz.org/area/e1c1215f-dcc0-35b4-b840-d2ca2151593b/releases?page=56 and http://test.musicbrainz.org/release/523ba1dc-a298-4229-95f9-dc86d15b0164

Elsewhere we display unknown values by not showing it at all, so this is inconsistent.