input: Write a function to compute the area of a circle given the radius. If the radius is less than or equal to zero, return `None`. Use the formula `A = π * r^2`.
  assert area_circle(5) == 78.54
  assert area_circle(10) == 314.16
  assert area_circle(0) == None
  assert area_circle(-5) == None
output: def area_circle(radius):
      if radius <= 0:
          return None
      return round(3.1416 * radius ** 2, 2)