input: Write a function to convert a given string, where each integer is separated by a comma, into a list of integers. 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("11,22,33,44,55")==[11, 22, 33, 44, 55]
  assert to_int_list("0,0,0,0")==[0, 0, 0, 0]
  assert to_int_list("100,200,300")==[100, 200, 300]
  assert to_int_list("")==[]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]