input: Write a function to calculate the sum of the items in a given list. If the list is empty, return 0.
  assert sum_list([1, 2, 3])==6
  assert sum_list([4, 5, 6, 7])==22
  assert sum_list([-1, -2, -3])==-6
  assert sum_list([10, -10])==0
  assert sum_list([])==0
output: def sum_list(lst):
      if not lst:
          return 0
      return sum(lst)