input: Write a function to calculate the area of a circle with the given radius. The function should return the area of the circle calculated using the formula: area = π * radius^2. If the radius is negative, return None.
  assert area_circle(5) == 78.54
  assert area_circle(10) == 314.16
  assert area_circle(0) == 0.0
  assert area_circle(-5) == None
output: def area_circle(radius):
      if radius < 0:
          return None
      return 3.1416 * radius ** 2