input: Write a function to convert a given string to a list of integers. The function should take a string as input, where the string consists of integers separated by commas, and return a list of integers.
  assert to_int_list("1,2,3,4,5") == [1, 2, 3, 4, 5]
  assert to_int_list("10,20,30") == [10, 20, 30]
  assert to_int_list("0,0,0") == [0, 0, 0]
  assert to_int_list("") == []
  assert to_int_list("1") == [1]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]