input: Write a function to find the index of the largest number in a given list of integers. If the list is empty, the function should return None.
  assert find_largest_index([5, 10, 20, 6, 8])==2
  assert find_largest_index([30, -5, 15, 25, 30])==4
  assert find_largest_index([-1, -2, -3, -4, -5])==0
  assert find_largest_index([1])==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)