Fix infinite querying when the user has no notifications or DMs

lib/friendica_client/pages_manager.dart

@@ -41,8 +41,10 @@ class PagesManager<TResult, TID> {
     }
     final result = await onRequest(PagingData(limit: limit));
     if (result.isSuccess) {
-      final newPage = result.value.map((data) => idMapper(data));
-      _pages.add(newPage);
+      if (result.value.previous != null || result.value.next != null) {
+        final newPage = result.value.map((data) => idMapper(data));
+        _pages.add(newPage);
+      }
     }
     return result;
   }

lib/screens/notifications_screen.dart

@@ -41,7 +41,6 @@ class NotificationsScreen extends StatelessWidget {
         ),
       ];
       if (notifications.isEmpty) {
-        manager.updateNotifications();
         title = 'Notifications';
         body = Center(
             child: Column(

lib/services/direct_message_service.dart

@@ -14,10 +14,12 @@ import 'auth_service.dart';
 class DirectMessageService extends ChangeNotifier {
   static final _logger = Logger('$DirectMessageService');
   final _threads = <String, DirectMessageThread>{};
+  var _firstLoading = true;
 
   List<DirectMessageThread> getThreads({bool unreadyOnly = false}) {
-    if (_threads.isEmpty) {
+    if (_threads.isEmpty && _firstLoading) {
       updateThreads();
+      _firstLoading = false;
     }
 
     if (unreadyOnly) {

lib/services/notifications_manager.dart

@@ -22,8 +22,13 @@ class NotificationsManager extends ChangeNotifier {
     idMapper: (nn) => nn.map((n) => n.id).toList(),
     onRequest: _clientGetNotificationsRequest,
   );
+  var _firstLoad = true;
 
   List<UserNotification> get notifications {
+    if (_notifications.isEmpty && _firstLoad) {
+      updateNotifications();
+      _firstLoad = false;
+    }
     final result = List<UserNotification>.from(_notifications.values);
     result.sort((n1, n2) {
       if (n1.dismissed == n2.dismissed) {