input: Write a function to convert a given string to a list of integers. The input string will be a comma-separated sequence of integers. If the input string is empty, the function should return an empty list.
  assert to_int_list("1,2,3,4,5")==[1, 2, 3, 4, 5]
  assert to_int_list("10,-20,30")==[10, -20, 30]
  assert to_int_list("0")==[0]
  assert to_int_list("")==[]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]