input: Write a function to convert a given string to a list of integers. The input string consists of numbers separated by commas. If the input string is empty then return an empty list. 
  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("")==[]
  assert to_int_list("5,6,7,8,9")==[5, 6, 7, 8, 9]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]