input: Write a function to generate a list comprehension that returns the squared values of the given list `[1, 2, 3, 5, 8, 13]`. If the input list is not provided, the function should return an empty list.
  assert squared_list([1, 2, 3, 5, 8, 13]) == [1, 4, 9, 25, 64, 169]
  assert squared_list([]) == []
  assert squared_list() == []
output: def squared_list(lst=None):
      if lst is None:
          return []
      return [x**2 for x in lst]