What continent does not have any countries on it?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q6173448', 'id': 0, 'label': 'Wikipedia:Vital articles/Level/4'}, {'type': 'INTERNAL', 'name_': 'Q5', 'id': 1, 'label': 'human'}, {'type': 'INTERNAL', 'name_': 'Q51', 'id': 2, 'label': 'Antarctica'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q5107', 'id': 3, 'label': 'continent'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q2', 'id': 4, 'label': 'Earth'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q6256', 'id': 5, 'label': 'country'}], 'links': [{'name_': 'P5008', 'source': 1, 'target': 0, 'label': 'on focus list of Wikimedia project'}, {'name_': 'P129', 'source': 1, 'target': 4, 'label': 'physically interacts with'}, {'name_': 'P5008', 'source': 2, 'target': 0, 'label': 'on focus list of Wikimedia project'}, {'name_': 'P31', 'source': 2, 'target': 3, 'label': 'instance of'}, {'name_': 'P361', 'source': 2, 'target': 4, 'label': 'part of'}, {'name_': 'P5008', 'source': 5, 'target': 0, 'label': 'on focus list of Wikimedia project'}]}