input: Write a function to convert a given string to a list of integers, where the integers are separated by commas. If the input 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("")==[]
  assert to_int_list("0,1,0,1,0")==[0, 1, 0, 1, 0]
  assert to_int_list("100,200,300,400")==[100, 200, 300, 400]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(",")]