fix(PosterPage): 修复背景图片未定义时使用默认图片的逻辑

 <!--
  * @Date: 2025-12-23 15:50:59
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-12-25 13:07:01
+ * @LastEditTime: 2025-12-25 13:22:42
  * @FilePath: /mlaj/src/views/recall/PosterPage.vue
  * @Description: 分享海报页面
 -->
@@ -174,7 +174,7 @@ onMounted(async () => {
     })
 
     if (code) {
-      posterBg.value = data.background_image + '?imageMogr2/thumbnail/800x/strip/quality/80' || defaultBg
+      posterBg.value = data.background_image ? data.background_image + '?imageMogr2/thumbnail/800x/strip/quality/80' : defaultBg
       qrCodeUrl.value = data?.qrcode || ''
       isReady.value = true
     }