input: Write a function that given a width and a length returns a Rectangle object that represents a two-dimensional rectangle. The object should have a method called area that returns the area of the rectangle which is the product of its length and width. If a width or a length less than or equal to zero is given, return None.
  assert Rectangle(5, 10).area()==50
  assert Rectangle(10, 10).area()==100
  assert Rectangle(-1, 10)==None
  assert Rectangle(0, 10)==None
  assert Rectangle(5, 0)==None
output: class Rectangle:
      def __init__(self, width, length):
          if width <= 0 or length <= 0:
              return None
          self.width = width
          self.length = length
  
      def area(self):
          return self.width * self.length