input: Write a function to calculate the odds ratio from a contingency table.
  assert odds_ratio([[100, 75], [40, 35]]) == (100 * 35) / (75 * 40)
  assert odds_ratio([[10, 20], [30, 40]]) == (10 * 40) / (20 * 30)
  assert odds_ratio([[50, 10], [20, 5]]) == (50 * 5) / (10 * 20)
  assert odds_ratio([[1, 2], [0, 1]]) == None
output: def odds_ratio(table):
      a, b = table[0]
      c, d = table[1]
      if b == 0 or c == 0:
          return None
      return (a * d) / (b * c)