input: Write a function to generate a Python program that adds two numbers and then doubles the result. If either of the input numbers is None, the function should return None.
  def double_add(num1, num2):
      if num1 is None or num2 is None:
          return None
      return (num1 + num2) * 2
  
  assert double_add(3, 4) == 14
  assert double_add(5, 2) == 14
  assert double_add(-1, 1) == 0
  assert double_add(0, 0) == 0
  assert double_add(None, 2) == None
  assert double_add(3, None) == None
output: def double_add(num1, num2):
      if num1 is None or num2 is None:
          return None
      return (num1 + num2) * 2