empty method
Implementation
BoundingSphere empty() {
  center.x = 0;
  center.y = 0;
  center.z = 0;
  radius = double.infinity;
  return this;
}BoundingSphere empty() {
  center.x = 0;
  center.y = 0;
  center.z = 0;
  radius = double.infinity;
  return this;
}