Not having 'relation_list' is not invalid, do not emit error message

Error message is still emitted if metadata has no artist (which shouldn't happen for artist query)

E: 17:27:06 Album Artist Website: u'2451caed-5d09-456f-b640-5880b6060a64': MusicBrainz artist xml result not in correct format - {'attribs': {}, 'text': u'', 'children': {u'metadata': [{'attribs': {}, 'text': u'', 'children': {u'artist': [{'attribs': {u'type': u'Group', u'id': u'2451caed-5d09-456f-b640-5880b6060a64'}, 'text': u'', 'children': {u'name': [{'attribs': {}, 'text': u'Sir Frankie Crisp', 'children': {}}], u'sort_name': [{'attribs': {}, 'text': u'Sir Frankie Crisp', 'children': {}}]}}]}}]}}