input: Write a function to read in 15 integers and return their sum. If the input list is empty, return 0.
  assert sum_nums([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14])==105
  assert sum_nums([])==0
output: def sum_nums(nums):
      if not nums:
          return 0
      return sum(nums)