msglist [nfc]: Represent nobody-is-typing as null, not empty list

Just to skip the bit of extra work of creating new lists.

Author: chrisbobbe

lib/model/typing_status.dart

@@ -21,8 +21,8 @@ class TypingStatus extends PerAccountStoreBase with ChangeNotifier {
 
   Iterable<SendableNarrow> get debugActiveNarrows => _timerMapsByNarrow.keys;
 
-  Iterable<int> typistIdsInNarrow(SendableNarrow narrow) =>
-    _timerMapsByNarrow[narrow]?.keys ?? [];
+  Iterable<int>? typistIdsInNarrow(SendableNarrow narrow) =>
+    _timerMapsByNarrow[narrow]?.keys;
 
   // Using SendableNarrow as the key covers the narrows
   // where typing notices are supported (topics and DMs).

lib/widgets/message_list.dart

@@ -1012,7 +1012,7 @@ class _TypingStatusWidgetState extends State<TypingStatusWidget> with PerAccount
     final store = PerAccountStoreWidget.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
     final typistIds = model!.typistIdsInNarrow(narrow);
-    if (typistIds.isEmpty) return const SizedBox();
+    if (typistIds == null) return const SizedBox();
     final text = switch (typistIds.length) {
       1 => zulipLocalizations.onePersonTyping(
              store.userDisplayName(typistIds.first)),

test/model/typing_status_test.dart

@@ -65,7 +65,7 @@ void main() {
   }
 
   void checkTypists(Map<SendableNarrow, List<User>> typistsByNarrow) {
-    final actualTypistsByNarrow = <SendableNarrow, Iterable<int>>{};
+    final actualTypistsByNarrow = <SendableNarrow, Iterable<int>?>{};
     for (final narrow in model.debugActiveNarrows) {
       actualTypistsByNarrow[narrow] = model.typistIdsInNarrow(narrow);
     }