input: Write a function to convert a given string into a list of integers by splitting the string on commas. If the given 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")==[10, 20, 30]
  assert to_int_list("0,0,0")==[0, 0, 0]
  assert to_int_list("")==[]
  assert to_int_list("5,3,8,1")==[5, 3, 8, 1]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]