input: Write a function to find the index of the largest number in a list of integers. If the list is empty, return None.
  
  assert find_largest_index([5, 10, 20, 6, 8]) == 2
  assert find_largest_index([3, 5, 4, 1, 2]) == 1
  assert find_largest_index([3, 3, 3, 3, 3]) == 0
  assert find_largest_index([]) == None
output: def find_largest_index(lst):
      if not lst:
          return None
      max_val = max(lst)
      return lst.index(max_val)