input: Write a function to convert a given string into a list of integers. The string is expected to contain numbers separated by commas. If the given string is empty then 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("1,2,a")==[1, 2, 'a']
  assert to_int_list("3,4,-5,6")==[3, 4, -5, 6]
output: def to_int_list(s):
      if not s:
          return []
      return [int(x) if x.isdigit() or (x[0] == '-' and x[1:].isdigit()) else x for x in s.split(',')]