msglist [nfc]: Use const to avoid making new lists for nobody-is-typing

Author: chrisbobbe

lib/model/typing_status.dart

@@ -22,7 +22,7 @@ class TypingStatus extends PerAccountStoreBase with ChangeNotifier {
   Iterable<SendableNarrow> get debugActiveNarrows => _timerMapsByNarrow.keys;
 
   Iterable<int> typistIdsInNarrow(SendableNarrow narrow) =>
-    _timerMapsByNarrow[narrow]?.keys ?? [];
+    _timerMapsByNarrow[narrow]?.keys ?? const [];
 
   // Using SendableNarrow as the key covers the narrows
   // where typing notices are supported (topics and DMs).