input: Write a function to find the first non-repeating character in a given list. If all characters are repeating, return None. If the list is empty, return None.
  assert first_non_repeating_char([1, 2, 3, 4, 4, 3, 2, 1])==None
  assert first_non_repeating_char([1, 2, 3, 4, 5])==1
  assert first_non_repeating_char([2, 2, 3, 3, 4, 4])==None
  assert first_non_repeating_char([1, 1, 2, 2, 3, 3, 4])==4
  assert first_non_repeating_char([])==None
output: def first_non_repeating_char(lst):
      if not lst:
          return None
      count = {}
      for item in lst:
          if item in count:
              count[item] += 1
          else:
              count[item] = 1
      for item in lst:
          if count[item] == 1:
              return item
      return None