input: Write a function to generate a countdown sequence from a given positive integer using a while loop and print each number counting down to 1, then print "Done!". If the input number is not positive, return an empty string.
  def countdown(num):
      if num <= 0:
          return ""
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  assert countdown(6)==""
  assert countdown(0)==""
  assert countdown(3)==None
output: def countdown(num):
      if num <= 0:
          return ""
      while num > 0:
          print(num)
          num -= 1
      print("Done!")