docs: replace empty()

Using empty() is not a good practice.

Author: kenjis

user_guide_src/source/tutorial/news_section/005.php

@@ -1,6 +1,6 @@
 <h2><?= esc($title) ?></h2>
 
-<?php if (! empty($news) && is_array($news)): ?>
+<?php if ($news !== []): ?>
 
     <?php foreach ($news as $news_item): ?>
 

user_guide_src/source/tutorial/news_section/006.php

@@ -15,7 +15,7 @@ public function show($slug = null)
 
         $data['news'] = $model->getNews($slug);
 
-        if (empty($data['news'])) {
+        if ($data['news'] === null) {
             throw new PageNotFoundException('Cannot find the news item: ' . $slug);
         }