input: Write a function to convert a given string to a list of integers by splitting the string at commas and converting each split part to an integer.
  assert to_int_list("1,2,3,4,5")==[1, 2, 3, 4, 5]
  assert to_int_list("10,20,30,40")==[10, 20, 30, 40]
  assert to_int_list("100,200,300")==[100, 200, 300]
  assert to_int_list("5,10,15,20")==[5, 10, 15, 20]
  assert to_int_list("")==[]
output: def to_int_list(s):
      return [int(x) for x in s.split(',') if x]