fix: fail to save existing post with blank content (#2615)

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#### What type of PR is this?
/kind bug
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#### What this PR does / why we need it:

保存空白内容的草稿时，将 `null` 转换为无内容的字符串，防止读取相关实体的数据时发生空指针异常

#### Which issue(s) this PR fixes:

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Fixes #2614

#### Special notes for your reviewer:

#### Does this PR introduce a user-facing change?

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src/main/java/run/halo/app/service/impl/ContentPatchLogServiceImpl.java

@@ -59,6 +59,13 @@ public class ContentPatchLogServiceImpl extends AbstractCrudService<ContentPatch
     @Override
     @Transactional(rollbackFor = Exception.class)
     public ContentPatchLog createOrUpdate(Integer postId, String content, String originalContent) {
+        if (originalContent == null) {
+            originalContent = "";
+        }
+        if (content == null) {
+            content = "";
+        }
+
         Integer version = getVersionByPostId(postId);
         if (existDraftBy(postId)) {
             return updateDraftBy(postId, content, originalContent);