Rankings generator crashes when only one metric in precedence

[czlee]

BACKEND-VR: https://sentry.io/tabbycat-ig/backend/issues/645557194/

I think it crashes because when trying to generate a subrank, it looks for metrics[1:], which is an empty list whenever the team standings precedence has only one element in it.

TypeError: itemgetter expected 1 arguments, got 0
(9 additional frame(s) were not displayed)
...
  File "draw/manager.py", line 191, in get_teams
    generator = TeamStandingsGenerator(metrics, ('rank', 'subrank'), tiebreak="random")
  File "standings/base.py", line 294, in __init__
    self._interpret_rankings(rankings)
  File "standings/base.py", line 393, in _interpret_rankings
    annotator = klass(self.precedence)
  File "standings/ranking.py", line 97, in __init__
    self.rank_key = metricgetter(*metrics[1:])
  File "standings/metrics.py", line 23, in metricgetter
    metricitemgetter = itemgetter(*items)

TypeError: itemgetter expected 1 arguments, got 0