input: Write a function to add two numbers and then double the result. If either of the inputs is None, return None.
  assert double_add(2, 3) == 10
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(None, 5) == None
  assert double_add(5, None) == None
  assert double_add(None, None) == None
output: def double_add(a, b):
      if a is None or b is None:
          return None
      return (a + b) * 2