input: Write a function to convert a given string to a list of integers. If the given string is empty then return an empty list.
  def to_int_list(s):
      if not s:
          return []
      return [int(x) for x in s.split(',')]
  assert to_int_list("1,2,3")==[1, 2, 3]
  assert to_int_list("4,5,6")==[4, 5, 6]
  assert to_int_list("")==[]
  assert to_int_list("7,8,9")==[7, 8, 9]
  assert to_int_list("10,11,12")==[10, 11, 12]
output: def to_int_list(s):
      if not s:
          return []
      return [int(x) for x in s.split(',')]