input: Write a function to convert a given string to a list of integers. The string is expected to contain integers separated by commas. If the string is empty, return an empty list.
  assert to_int_list("1,2,3")==[1, 2, 3]
  assert to_int_list("4,5,6,7")==[4, 5, 6, 7]
  assert to_int_list("")==[]
  assert to_int_list("10,20,30")==[10, 20, 30]
  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(',')]