chore: demo spec uses ts (#8197)

test/demo.ts

@@ -1,14 +1,14 @@
-import { h } from 'vue';
+import { h, defineComponent } from 'vue';
 import Locale from '../src/locale';
 import { mount, later } from '.';
 import { DemoLocaleMixin } from '../docs/site/demo-locale';
 
-const EmptyComponent = {
-  render() {
-    return h('div', [this.$slots.default()]);
-  },
+const EmptyComponent = defineComponent({
   inheritAttrs: false,
-};
+  render() {
+    return h('div', [this.$slots.default?.()]);
+  },
+});
 
 export function snapshotDemo(Demo: any, option: any = {}) {
   test('should render demo and match snapshot', async () => {