input: Write a function that enhances the given Python program by adding a positive feedback loop. The function should take as input a loop that prints "hello" 10 times, and modify it to also print a congratulatory message when the loop completes. If the given input is empty, the function should return None.
  assert design_feedback_loop("for i in range(10):\n    print('hello')") == "for i in range(10):\n    print('hello')\n    i += 1\n    if i == 10:\n        print('You've got it!')"
  assert design_feedback_loop("") == None
output: def design_feedback_loop(loop_code):
      if not loop_code:
          return None
      return loop_code + "\n    i += 1\n    if i == 10:\n        print('You've got it!')"