inbox [nfc]: Display realmEmptyTopicDisplayName where empty topic appears

While this appears to be a user facing change, it's not visible yet, not
until TopicName.displayName becomes nullable.  This is a part of a
series of changes to handle empty topics.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/inbox.dart

@@ -532,12 +532,15 @@ class _TopicItem extends StatelessWidget {
                 style: TextStyle(
                   fontSize: 17,
                   height: (20 / 17),
+                  // ignore: unnecessary_null_comparison // null topic names soon to be enabled
+                  fontStyle: topic.displayName == null ? FontStyle.italic : null,
                   // TODO(design) check if this is the right variable
                   color: designVariables.labelMenuButton,
                 ),
                 maxLines: 2,
                 overflow: TextOverflow.ellipsis,
-                topic.displayName))),
+                // ignore: dead_null_aware_expression // null topic names soon to be enabled
+                topic.displayName ?? store.realmEmptyTopicDisplayName))),
             const SizedBox(width: 12),
             if (hasMention) const _IconMarker(icon: ZulipIcons.at_sign),
             // TODO(design) copies the "@" marker color; is there a better color?

test/widgets/inbox_test.dart

@@ -307,6 +307,15 @@ void main() {
       });
     });
 
+    testWidgets('empty topic', (tester) async {
+      final channel = eg.stream();
+      await setupPage(tester,
+        streams: [channel],
+        subscriptions: [(eg.subscription(channel))],
+        unreadMessages: [eg.streamMessage(stream: channel, topic: '')]);
+      check(find.text(eg.defaultRealmEmptyTopicDisplayName)).findsOne();
+    }, skip: true); // null topic names soon to be enabled
+
     group('topic visibility', () {
       final channel = eg.stream();
       const topic = 'topic';