do not merge; compose: Handle hint text for empty topics

This is a placeholder before we actually handle empty topics for compose
box in general.

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

Author: PIG208

lib/widgets/compose_box.dart

@@ -586,6 +586,10 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
     final topic = TopicName(widget.controller.topic.textNormalized);
+    // Until we support sending to empty topics, `topic.displayName` will remain
+    // non-null because the topic input text gets normalized to "(no topic)".
+    // ignore: unnecessary_null_comparison // null topic names soon to be enabled
+    assert(topic.displayName != null);
     return _ContentInput(
       narrow: widget.narrow,
       destination: TopicNarrow(widget.narrow.streamId, topic),
@@ -658,7 +662,8 @@ 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 {