USDZExporter: add defaultPrim to make usdchecker happy (#26495)

examples/jsm/exporters/USDZExporter.js

@@ -165,6 +165,7 @@ function buildHeader() {
 	customLayerData = {
 		string creator = "Three.js USDZExporter"
 	}
+	defaultPrim = "Root"
 	metersPerUnit = 1
 	upAxis = "Y"
 )