Adjust compose hints toward "general chat"

lib/widgets/compose_box.dart

@@ -579,23 +579,39 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     super.dispose();
   }
 
+  /// The topic name to show in the hint text, or null to show no topic.
+  TopicName? _hintTopic() {
+    if (widget.controller.topic.isTopicVacuous) {
+      if (widget.controller.topic.mandatory) {
+        // The chosen topic can't be sent to, so don't show it.
+        return null;
+      }
+    }
+
+    return TopicName(widget.controller.topic.textNormalized);
+  }
+
   @override
   Widget build(BuildContext context) {
     final store = PerAccountStoreWidget.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
+
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
-    final topic = TopicName(widget.controller.topic.textNormalized);
+    final hintTopic = _hintTopic();
+    final hintDestination = hintTopic == 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
+      '#$streamName' : '#$streamName > ${hintTopic.displayName}';
+
     return _ContentInput(
       narrow: widget.narrow,
-      destination: TopicNarrow(widget.narrow.streamId, topic),
+      destination: TopicNarrow(widget.narrow.streamId,
+        TopicName(widget.controller.topic.textNormalized)),
       controller: widget.controller,
-      hintText: zulipLocalizations.composeBoxChannelContentHint(
-        // No i18n of this use of "#" and ">" string; those are part of how
-        // Zulip expresses channels and topics, not any normal English punctuation,
-        // so don't make sense to translate. See:
-        //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
-        '#$streamName > ${topic.displayName}'));
+      hintText: zulipLocalizations.composeBoxChannelContentHint(hintDestination));
   }
 }