input: Write a function to calculate the sum of the items in a given list. If the given list is empty then return 0.
  assert sum_list([1, 2, 3, 4])==10
  assert sum_list([5, 10, 15, 20])==50
  assert sum_list([100, 200, 300])==600
  assert sum_list([])==0
output: def sum_list(lst):
      if not lst:
          return 0
      return sum(lst)