input: Write a function to convert a given string into a list of integers. If the string is empty, return an empty list.
  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("5")==[5]
  assert to_int_list("")==[]
  assert to_int_list("1,2,3,a")==[1, 2, 3]
output: def to_int_list(s):
      if not s:
          return []
      try:
          return [int(x) for x in s.split(',')]
      except ValueError:
          return [int(x) for x in s.split(',') if x.isdigit()]