input: Write a function to generate the first n elements of the Sierpinski triangle.
  
  assert sierpinski_triangle(6)==None
  assert sierpinski_triangle(3)==None
  assert sierpinski_triangle(0)==None
  assert sierpinski_triangle(8)==None
output: def sierpinski_triangle(n):
      if n <= 0:
          return None
      triangle = ["*"]
      for i in range(1, n):
          new_row = ""
          for j in range(len(triangle[0])):
              if j == 0 or j == len(triangle[0]) - 1:
                  new_row += "*"
              else:
                  new_row += " "
          triangle.append(new_row)
          triangle[0] = "*" + triangle[0] + "*"
      for row in triangle:
          print(row)
      return None