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 not provided, the function should still return the correct result.
  assert double_add(3, 4)==14
  assert double_add(10, 5)==30
  assert double_add(0, 0)==0
  assert double_add(1, None)==2
  assert double_add(None, 1)==2
output: def double_add(a=0, b=0):
      return (a + b) * 2