compose: Unskip test; show realmEmptyDisplayName in hint text

This logic was introduced in 769cc7d, which assumed that
TopicName.displayName is `null` when the topic is empty.
TopicName that came from the server are guaranteed to be non-empty,
but here our code can construct an empty TopicName, breaking this
assumption.

Switch to using plain strings, and go back to constructing TopicName
with empty topics once TopicName.displayName becomes nullable.

Author: PIG208

lib/widgets/compose_box.dart

@@ -624,7 +624,7 @@ class _StreamContentInputState extends State<_StreamContentInput> {
   }
 
   /// The topic name to show in the hint text, or null to show no topic.
-  TopicName? _hintTopic() {
+  String? _hintTopicStr() {
     if (widget.controller.topic.isTopicVacuous) {
       if (widget.controller.topic.mandatory) {
         // The chosen topic can't be sent to, so don't show it.
@@ -639,7 +639,7 @@ class _StreamContentInputState extends State<_StreamContentInput> {
       }
     }
 
-    return TopicName(widget.controller.topic.textNormalized);
+    return widget.controller.topic.textNormalized;
   }
 
   @override
@@ -649,15 +649,15 @@ class _StreamContentInputState extends State<_StreamContentInput> {
 
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
-    final hintTopic = _hintTopic();
-    final hintDestination = hintTopic == null
+    final hintTopicStr = _hintTopicStr();
+    final hintDestination = hintTopicStr == null
       // No i18n of this use of "#" and ">" string; those are part of how
       // 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
       ? '#$streamName'
-      // ignore: dead_null_aware_expression // null topic names soon to be enabled
-      : '#$streamName > ${hintTopic.displayName ?? store.realmEmptyTopicDisplayName}';
+      : '#$streamName > '
+        '${hintTopicStr.isEmpty ? store.realmEmptyTopicDisplayName : hintTopicStr}';
 
     return _TypingNotifier(
       destination: TopicNarrow(widget.narrow.streamId,

test/widgets/compose_box_test.dart

@@ -404,7 +404,7 @@ void main() {
           topicHintText: 'Topic',
           contentHintText: 'Message #${channel.name} > '
                            '${eg.defaultRealmEmptyTopicDisplayName}');
-      }, skip: true); // null topic names soon to be enabled
+      });
 
       testWidgets('legacy: with empty topic, content input has focus', (tester) async {
         await prepare(tester, narrow: narrow, mandatoryTopics: false,