ColladaLoader2: Support 0 opacity models. You never know...?

examples/js/loaders/ColladaLoader2.js

@@ -629,7 +629,7 @@ THREE.ColladaLoader.prototype = {
 							material.emissive.fromArray( parameter.color );
 						break;
 					case 'transparency':
-						if ( parameter.float ) material.opacity = parameter.float;
+						if ( parameter.float !== undefined ) material.opacity = parameter.float;
 						material.transparent = true;
 						break;