input: Write a function to generate the first n elements of the Sierpinski triangle in Python. 
  
  assert sierpinski_triangle(6)==None 
  assert sierpinski_triangle(3)==None 
  assert sierpinski_triangle(1)==None 
  assert sierpinski_triangle(0)==None
output: def sierpinski_triangle(n):
      if n <= 0:
          return None
      triangle = ["*"]
      for i in range(n):
          space = " " * (2 ** i)
          triangle = [space.join(line) for line in triangle] + [line + " " + line for line in triangle]
      for line in triangle:
          print(line)