input: Write a function to convert a given string to a list of integers. If the string is empty, return an empty list. Split the string by commas to get the list of integers.
  assert to_int_list("1,2,3,4")==[1, 2, 3, 4]
  assert to_int_list("5,6,7,8,9")==[5, 6, 7, 8, 9]
  assert to_int_list("10,11,12")==[10, 11, 12]
  assert to_int_list("")==[]
  assert to_int_list("1,2,3,a")==[1, 2, 3, 'a']
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) if x.isdigit() else x for x in s.split(',')]