input: Write a function to create a Python class called Rectangle that represents a two-dimensional rectangle with a length and width. Include a method to calculate the area of the rectangle. If the input length or width is negative, return None.
  class Rectangle:
      def __init__(self, length, width):
          if length < 0 or width < 0:
              return None
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width
  
  assert Rectangle(4, 5).area() == 20
  assert Rectangle(0, 7).area() == 0
  assert Rectangle(-3, 5) == None
  assert Rectangle(6, -2) == None
output: class Rectangle:
      def __init__(self, length, width):
          if length < 0 or width < 0:
              return None
          self.length = length
          self.width = width
  
      def area(self):
          return self.length * self.width