runtime(todo): Update todo.txt to remove recently addressed issues (#12910)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vim9.txt b/runtime/doc/vim9.txt
index 4b0cdbb..1431a13 100644
--- a/runtime/doc/vim9.txt
+++ b/runtime/doc/vim9.txt
@@ -1033,10 +1033,12 @@
 	null
 	null_blob
 	null_channel
+	null_class
 	null_dict
 	null_function
 	null_job
 	null_list
+	null_object
 	null_partial
 	null_string
 `true` is the same as `v:true`, `false` the same as `v:false`, `null` the same