submessage: Accept todo submessage with extra_data: null.

Before commit 6df3ad251a (#24235),
get_extra_data_from_widget_type would initialize extra_data to null
for todo messages.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/src/submessage.ts

@@ -42,7 +42,7 @@ const widget_data_event_schema = z.object({
         }),
         z.object({
             widget_type: z.literal("todo"),
-            extra_data: todo_widget_extra_data_schema,
+            extra_data: z.nullable(todo_widget_extra_data_schema),
         }),
     ]),
 });