docs: update upload demo ts

示例代码在运行时,ts类型报错.原因是试图在可能为 undefined 的对象上使用扩展运算符.

Author: huyikai

components/upload/demo/upload-manually.vue

@@ -57,7 +57,7 @@ export default defineComponent({
     };
 
     const beforeUpload: UploadProps['beforeUpload'] = file => {
-      fileList.value = [...fileList.value, file];
+      fileList.value = [...(fileList.value || []), file];
       return false;
     };