input: Write a function to convert a given string to a list of integers by splitting the string at each comma and converting each element 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("")==[]
  assert to_int_list("10,20,30,40,50")==[10, 20, 30, 40, 50]
  assert to_int_list("0,0,0")==[0, 0, 0]
output: def to_int_list(s):
      if s == "":
          return []
      return [int(x) for x in s.split(',')]