fix: resolve the issue of missing old avatar (#4293)

#### What type of PR is this?

/kind bug
/area core

#### What this PR does / why we need it:

解决 #4253  所导致的用户旧头像被删除的问题。

#### Special notes for your reviewer:

1. 使用 2.7.x 之前的 Halo 版本，新建用户并设置其头像。
2. 升级 Halo 至 2.8.0-rc.1 
3. 头像还存在即可。
4. 测试头像上传、移除功能是否正常可用。

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

```release-note
None
```

application/src/main/java/run/halo/app/core/extension/endpoint/UserEndpoint.java

@@ -209,6 +209,7 @@ public class UserEndpoint implements CustomEndpoint {
             .flatMap(user -> {
                 MetadataUtil.nullSafeAnnotations(user)
                     .remove(User.AVATAR_ATTACHMENT_NAME_ANNO);
+                user.getSpec().setAvatar(null);
                 return client.update(user);
             })
             .flatMap(user -> ServerResponse.ok().bodyValue(user));

application/src/main/java/run/halo/app/core/extension/reconciler/UserReconciler.java

@@ -78,12 +78,6 @@ public class UserReconciler implements Reconciler<Request> {
                 oldAvatarAttachmentName = null;
             }
 
-            if (StringUtils.isBlank(oldAvatarAttachmentName)
-                && StringUtils.isNotBlank(avatarAttachmentName)) {
-                oldAvatarAttachmentName = avatarAttachmentName;
-                annotations.put(User.LAST_AVATAR_ATTACHMENT_NAME_ANNO, oldAvatarAttachmentName);
-            }
-
             if (StringUtils.isNotBlank(avatarAttachmentName)) {
                 client.fetch(Attachment.class, avatarAttachmentName)
                     .ifPresent(attachment -> {
@@ -93,9 +87,15 @@ public class UserReconciler implements Reconciler<Request> {
                         }
                         user.getSpec().setAvatar(avatarUri.toString());
                     });
-            } else {
+            } else if (StringUtils.isNotBlank(oldAvatarAttachmentName)) {
                 user.getSpec().setAvatar(null);
             }
+
+            if (StringUtils.isBlank(oldAvatarAttachmentName)
+                && StringUtils.isNotBlank(avatarAttachmentName)) {
+                annotations.put(User.LAST_AVATAR_ATTACHMENT_NAME_ANNO, avatarAttachmentName);
+            }
+
             client.update(user);
         });
     }