compose [nfc]: Handle empty topics for topic-narrow input hint text

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.

A test is skipped because the server does not send empty topics to the
client without "empty_topic_name" client capability.

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

Author: PIG208

lib/widgets/compose_box.dart

@@ -658,7 +658,8 @@ class _FixedDestinationContentInput extends StatelessWidget {
           // Zulip expresses channels and topics, not any normal English punctuation,
           // so don't make sense to translate. See:
           //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
-          '#$streamName > ${topic.displayName}');
+          // ignore: dead_null_aware_expression // null topic names soon to be enabled
+          '#$streamName > ${topic.displayName ?? store.realmEmptyTopicDisplayName}');
 
       case DmNarrow(otherRecipientIds: []): // The self-1:1 thread.
         return zulipLocalizations.composeBoxSelfDmContentHint;

test/widgets/compose_box_test.dart

@@ -370,11 +370,20 @@ void main() {
       });
     });
 
-    testWidgets('to TopicNarrow', (tester) async {
-      await prepare(tester,
-        narrow: TopicNarrow(channel.streamId, TopicName('topic')));
-      checkComposeBoxHintTexts(tester,
-        contentHintText: 'Message #${channel.name} > topic');
+    group('to TopicNarrow', () {
+      testWidgets('with non-empty topic', (tester) async {
+        await prepare(tester,
+          narrow: TopicNarrow(channel.streamId, TopicName('topic')));
+        checkComposeBoxHintTexts(tester,
+          contentHintText: 'Message #${channel.name} > topic');
+      });
+
+      testWidgets('with empty topic', (tester) async {
+        await prepare(tester,
+          narrow: TopicNarrow(channel.streamId, TopicName('')));
+        checkComposeBoxHintTexts(tester, contentHintText:
+          'Message #${channel.name} > ${eg.defaultRealmEmptyTopicDisplayName}');
+      }, skip: true); // null topic names soon to be enabled
     });
 
     testWidgets('to DmNarrow with self', (tester) async {