input: Write a python function to retrieve a value from a nested dictionary using a list of keys.
  assert nested_dict_value({"a": {"b": {"c": 1}}}, ["a", "b", "c"]) == 1
  assert nested_dict_value({"a": {"b": {"c": 1}}}, ["a", "b"]) == {"c": 1}
  assert nested_dict_value({"a": {"b": {"c": 1}}}, ["a", "d"]) == None
output: def nested_dict_value(d, keys):
      for key in keys:
          if isinstance(d, dict) and key in d:
              d = d[key]
          else:
              return None
      return d