input: Write a function to convert a given string to a list of integers.
  def to_int_list(s):
      return [int(x) for x in s.split(',')]
  
  assert to_int_list("1,2,3")==[1, 2, 3]
  assert to_int_list("10,20,30,40")==[10, 20, 30, 40]
  assert to_int_list("4,8,15,16,23,42")==[4, 8, 15, 16, 23, 42]
  assert to_int_list("")==[]
  assert to_int_list("1")==[1]
output: def to_int_list(s):
      return [int(x) for x in s.split(',') if x]