fix: issue of post details page without meta_description variables (#2282)

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#### What type of PR is this?
/kind bug
/area core
/milestone 1.5.x
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#### What this PR does / why we need it:
修复文章内容查询问题导致 meta_description 内容生成为空
#### Which issue(s) this PR fixes:

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Fixes #2281 

#### Special notes for your reviewer:
/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

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```release-note
修复文章内容查询问题导致的文章详情页 meta_description 为空 
```

src/main/java/run/halo/app/controller/content/model/PostModel.java

@@ -23,6 +23,7 @@ import run.halo.app.model.entity.Tag;
 import run.halo.app.model.enums.EncryptTypeEnum;
 import run.halo.app.model.enums.PostStatus;
 import run.halo.app.model.vo.ArchiveYearVO;
+import run.halo.app.model.vo.PostDetailVO;
 import run.halo.app.model.vo.PostListVO;
 import run.halo.app.service.CategoryService;
 import run.halo.app.service.OptionService;
@@ -143,21 +144,25 @@ public class PostModel {
                 tags.stream().map(Tag::getName).collect(Collectors.joining(",")));
         }
 
+        model.addAttribute("is_post", true);
+
+        PostDetailVO postDetail;
+        if (StringUtils.isNotBlank(token)) {
+            postDetail = postRenderAssembler.convertToPreviewDetailVo(post);
+            model.addAttribute("post", postDetail);
+        } else {
+            postDetail = postRenderAssembler.convertToDetailVo(post);
+            model.addAttribute("post", postDetail);
+        }
+
         // Generate meta description.
         if (StringUtils.isNotEmpty(post.getMetaDescription())) {
             model.addAttribute("meta_description", post.getMetaDescription());
         } else {
             model.addAttribute("meta_description",
-                postService.generateDescription(post.getContent().getContent()));
+                postService.generateDescription(postDetail.getContent()));
         }
 
-        model.addAttribute("is_post", true);
-
-        if (StringUtils.isNotBlank(token)) {
-            model.addAttribute("post", postRenderAssembler.convertToPreviewDetailVo(post));
-        } else {
-            model.addAttribute("post", postRenderAssembler.convertToDetailVo(post));
-        }
 
         model.addAttribute("categories", categoryService.convertTo(categories));
         model.addAttribute("tags", tagService.convertTo(tags));