Reader: Preserve editorial date on post detail refresh (#22783)

* Reader: Preserve editorial date when detail endpoint refreshes post

When the Reader detail screen fetches a post, the response lacks the
editorial section, causing the raw date field to overwrite the correct
Freshly Pressed displayed_on date. Preserve the existing date_published
from the DB before saving.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* Reader: Remove redundant isEmpty check on preserved date

getDatePublished() already returns a non-null string via
StringUtils.notNullStr(), making the TextUtils.isEmpty guard
unnecessary.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: nbradbury

WordPress/src/main/java/org/wordpress/android/ui/reader/actions/ReaderPostActions.java

@@ -293,6 +293,19 @@ public void onResponse(JSONObject jsonObject) {
                 new Thread(() -> {
                     ReaderPost post = ReaderPost.fromJson(jsonObject);
 
+                    // Preserve the existing date_published if the post
+                    // already exists in the DB. This prevents editorial
+                    // dates (e.g. Freshly Pressed "displayed_on") from
+                    // being overwritten when the detail endpoint response
+                    // lacks the editorial section.
+                    ReaderPost existingPost =
+                            ReaderPostTable.getBlogPost(
+                                    post.blogId, post.postId, true);
+                    if (existingPost != null) {
+                        post.setDatePublished(
+                                existingPost.getDatePublished());
+                    }
+
                     ReaderPostTable.addPost(post);
                     handlePostLikes(post, jsonObject);