input: Write a function to calculate the sum of a given list of numbers. If the list is empty, return 0.
  assert sum_list([1, 2, 3, 4, 5])==15
  assert sum_list([10, 20, 30])==60
  assert sum_list([-1, -2, -3])==-6
  assert sum_list([])==0
output: def sum_list(numbers):
      if not numbers:
          return 0
      return sum(numbers)