input: Write a function to add two numbers and then double the result. If the input numbers are empty, return 0.
  assert double_add(3, 5)==16
  assert double_add(4, 4)==16
  assert double_add(6, 2)==16
  assert double_add(-1, 2)==2
  assert double_add(0, 0)==0
output: def double_add(a, b):
      if a is None or b is None:
          return 0
      return (a + b) * 2