harbour: Improve previous data access in API calls

packages/harbour/lib/src/apps/news/blocs/articles.dart

@@ -157,7 +157,7 @@ class NewsArticlesBloc extends $NewsArticlesBloc {
             getRead: getRead,
           ))!,
           (final response) => response.items,
-          previousData: _articlesSubject.hasValue ? _articlesSubject.value.data : null,
+          previousData: _articlesSubject.valueOrNull?.data,
         )
         .listen(_articlesSubject.add);
   }

packages/harbour/lib/src/apps/news/blocs/news.dart

@@ -187,7 +187,7 @@ class NewsBloc extends $NewsBloc {
           'news-folders',
           () async => (await client.news.listFolders())!,
           (final response) => response.folders,
-          previousData: _foldersSubject.hasValue ? _foldersSubject.value.data : null,
+          previousData: _foldersSubject.valueOrNull?.data,
         )
         .listen(_foldersSubject.add);
   }
@@ -203,7 +203,7 @@ class NewsBloc extends $NewsBloc {
         _newestItemId = response.newestItemId;
         return response.feeds;
       },
-      previousData: _feedsSubject.hasValue ? _feedsSubject.value.data : null,
+      previousData: _feedsSubject.valueOrNull?.data,
     ).listen(_feedsSubject.add);
   }
 

packages/harbour/lib/src/apps/notes/blocs/notes.dart

@@ -77,7 +77,7 @@ class NotesBloc extends $NotesBloc {
           'notes-notes',
           () async => (await client.notes.getNotes())!,
           (final response) => response,
-          previousData: _notesSubject.hasValue ? _notesSubject.value.data : null,
+          previousData: _notesSubject.valueOrNull?.data,
         )
         .listen(_notesSubject.add);
   }