Changed object to optional parameter

You can create an empty BoxHelper this case is considered in the code.

docs/api/helpers/BoxHelper.html

@@ -40,7 +40,7 @@
 
 		<h3>[name]( [page:Object3D object], [page:Color color] )</h3>
 		<div>
-		[page:Object3D object]  -- the object3D to show the world-axis-aligned boundingbox.<br />
+		[page:Object3D object]  -- (optional) the object3D to show the world-axis-aligned boundingbox.<br />
 		[page:Color color] --  (optional) hexadecimal value that defines the box's color. Default is 0xffff00.<br /><br />
 
 		Creates a new wireframe box that bounds the passed object. Internally this uses [page:Box3.setFromObject]