input: Write a function to compute the area of a circle given its radius. If the radius is provided, return the computed area; if the radius is not provided or is zero, return 0.
  assert area_circle(5) == 78.54
  assert area_circle(0) == 0
  assert area_circle(-1) == 0
output: def area_circle(radius=None):
      if radius is None or radius <= 0:
          return 0
      return 3.14159 * radius * radius