fix(download): 更新下载文件链接判断逻辑

- 使用!empty()替代trim()来检查url字段，确保更准确的链接判断
- 保持对缩略图链接的回退功能不变

public/themes/website/1/en/demo/download.html

@@ -119,7 +119,7 @@
       <div class="d-list">
         {hcTaglib:contentPage mainField="*" subField="*"  name="hc_content" limit="12" item="vo" sort="sort desc"}
         {php}
-        $dl_file = trim((string)($vo['url'] ?? ($vo['thumbnail']['url'] ?? '')));
+        $dl_file = !empty($vo['url']) ? $vo['url'] : ($vo['thumbnail']['url'] ?? '');
         $has_dl = $dl_file !== '';
         {/php}
         <div class="d-item">

public/themes/website/1/zh/demo/download.html

@@ -119,7 +119,7 @@
       <div class="d-list">
         {hcTaglib:contentPage mainField="*" subField="*"  name="hc_content" limit="12" item="vo" sort="sort desc"}
         {php}
-        $dl_file = trim((string)($vo['url'] ?? ($vo['thumbnail']['url'] ?? '')));
+        $dl_file = !empty($vo['url']) ? $vo['url'] : ($vo['thumbnail']['url'] ?? '');
         $has_dl = $dl_file !== '';
         {/php}
         <div class="d-item">