input: Write a function to convert a given string to a list of integers by splitting the string by commas. If the string is empty, return an empty list.
  assert to_int_list("1,2,3,4,5")==[1, 2, 3, 4, 5]
  assert to_int_list("10,20,30,40")==[10, 20, 30, 40]
  assert to_int_list("")==[]
  assert to_int_list("5,15,25")==[5, 15, 25]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(",")]