input: Write a function to calculate the Gini coefficient of a given dataset. The function should take a list as an input and return the Gini coefficient as a float value. If the dataset is empty, return 0.
  assert gini([2,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,21])==0.605
  assert gini([1,2,3,4,5])==0.28
  assert gini([])==0
output: def gini(data):
      if not data:
          return 0
      n = len(data)
      data.sort()
      total = sum(data)
      gini_sum = 0
      for i, value in enumerate(data):
          gini_sum += (2 * (i + 1) - n - 1) * value
      return gini_sum / (n * total)