input: Write a function to calculate the sum of all elements in a given list. If the given list is empty then return 0.
  assert sum_list([1, 2, 3, 4, 5])==15
  assert sum_list([10, 20, 30])==60
  assert sum_list([-1, 0, 1])==0
  assert sum_list([])==0
output: def sum_list(lst):
      if not lst:
          return 0
      return sum(lst)