update persistence client test

Author: renefloor

packages/stream_chat/test/src/db/chat_persistence_client_test.dart

@@ -19,7 +19,7 @@ class TestPersistenceClient extends ChatPersistenceClient {
   bool get isConnected => throw UnimplementedError();
 
   @override
-  String? get userId => throw UnimplementedError();
+  String? get userId => 'test-user-id';
 
   @override
   Future<void> connect(String userId) => throw UnimplementedError();