compose: Unskip test; handle TopicName with empty displayName for now

This logic was introduced in 769cc7d, which relied on the fact that
TopicName.displayName is `null` when the topic is empty.  Normally it is
fine to assume that the displayName is non-empty with the server flags,
but in this case the TopicName is constructed by our code.

Author: PIG208

lib/widgets/compose_box.dart

@@ -642,8 +642,10 @@ class _StreamContentInputState extends State<_StreamContentInput> {
       // 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 > ${hintTopic.displayName ?? store.realmEmptyTopicDisplayName}';
+      // null topic names soon to be enabled
+      : '#$streamName > '
+        '${hintTopic.displayName.isEmpty ? store.realmEmptyTopicDisplayName
+                                         : hintTopic.displayName}';
 
     return _ContentInput(
       narrow: widget.narrow,

test/widgets/compose_box_test.dart

@@ -398,7 +398,7 @@ 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,