msglist: Display realmEmptyTopicDisplayName where empty topic appears

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

lib/widgets/message_list.dart

@@ -366,8 +366,11 @@ class MessageListAppBarTitle extends StatelessWidget {
     return Row(
       mainAxisSize: MainAxisSize.min,
       children: [
-        Flexible(child: Text(topic.displayName, style: const TextStyle(
+        // ignore: dead_null_aware_expression // null topic names soon to be enabled
+        Flexible(child: Text(topic.displayName ?? store.realmEmptyTopicDisplayName, style: TextStyle(
           fontSize: 13,
+          // ignore: unnecessary_null_comparison // null topic names soon to be enabled
+          fontStyle: topic.displayName == null ? FontStyle.italic : null,
         ).merge(weightVariableTextStyle(context)))),
         if (icon != null)
           Padding(
@@ -1116,11 +1119,15 @@ class StreamMessageRecipientHeader extends StatelessWidget {
       child: Row(
         children: [
           Flexible(
-            child: Text(topic.displayName,
+            // ignore: dead_null_aware_expression // null topic names soon to be enabled
+            child: Text(topic.displayName ?? store.realmEmptyTopicDisplayName,
               // TODO: Give a way to see the whole topic (maybe a
               //   long-press interaction?)
               overflow: TextOverflow.ellipsis,
-              style: recipientHeaderTextStyle(context))),
+              style: recipientHeaderTextStyle(context).copyWith(
+                // ignore: unnecessary_null_comparison // null topic names soon to be enabled
+                fontStyle: topic.displayName == null ? FontStyle.italic : null,
+              ))),
           const SizedBox(width: 4),
           // TODO(design) copies the recipient header in web; is there a better color?
           Icon(size: 14, color: designVariables.colorMessageHeaderIconInteractive,

test/widgets/message_list_test.dart

@@ -808,6 +808,26 @@ void main() {
         check(findInMessageList('topic name')).length.equals(1);
       });
 
+      final messageEmptyTopic = eg.streamMessage(stream: stream, topic: '');
+
+      testWidgets('show general chat for empty topics with channel name', (tester) async {
+        await setupMessageListPage(tester,
+          narrow: const CombinedFeedNarrow(),
+          messages: [messageEmptyTopic], subscriptions: [eg.subscription(stream)]);
+        await tester.pump();
+        check(findInMessageList('stream name')).length.equals(1);
+        check(findInMessageList(eg.defaultRealmEmptyTopicDisplayName)).length.equals(1);
+      }, skip: true); // null topic names soon to be enabled
+
+      testWidgets('show general chat for empty topics without channel name', (tester) async {
+        await setupMessageListPage(tester,
+          narrow: TopicNarrow.ofMessage(messageEmptyTopic),
+          messages: [messageEmptyTopic]);
+        await tester.pump();
+        check(findInMessageList('stream name')).length.equals(0);
+        check(findInMessageList(eg.defaultRealmEmptyTopicDisplayName)).length.equals(1);
+      }, skip: true); // null topic names soon to be enabled
+
       testWidgets('show topic visibility icon when followed', (tester) async {
         await setupMessageListPage(tester,
           narrow: const CombinedFeedNarrow(),