Don't warn if an app returns NULL when referencing a child

Don't warn if an app returns NULL when referencing a child

If atk_object_ref_accessible_child returns NULL when creating the cache,
then don't attempt to process/unref the NULL item, although applications
probably shouldn't be returning NULL in response to a call to
atk_object_ref_accessible_child, so it seems debatable whether we should
be printing a warning here.