Fixes setting of material reflectivity. (#8995)

src/loaders/MaterialLoader.js

@@ -126,7 +126,7 @@ Object.assign( THREE.MaterialLoader.prototype, {
 
 		}
 
-		if ( json.reflectivity ) material.reflectivity = json.reflectivity;
+		if ( json.reflectivity !== undefined ) material.reflectivity = json.reflectivity;
 
 		if ( json.lightMap !== undefined ) material.lightMap = this.getTexture( json.lightMap );
 		if ( json.lightMapIntensity !== undefined ) material.lightMapIntensity = json.lightMapIntensity;