input: Write a function that converts a given string to a list of integers. The function should split the string by commas and convert each of the resulting substrings to an integer. If the input string is empty, the function should return an empty list.
  assert to_int_list("1,2,3")==[1, 2, 3]
  assert to_int_list("4,5,6,7")==[4, 5, 6, 7]
  assert to_int_list("10,20")==[10, 20]
  assert to_int_list("")==[]
  assert to_int_list("3,4,5,6,7,8,9")==[3, 4, 5, 6, 7, 8, 9]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]