Default to comparing all attributes if no oo_id_attrs defined

Default to comparing all attributes if no oo_id_attrs defined

Since commit: "cache pickup: Avoid duplicates during cache pickup",
nfnl_ct_alloc_cache no longer properly fills the cache, as it doesn't
define oo_id_attrs so all items are considered duplicates.

Instead of adding a ~0 oo_id_attrs to ct_obj, this changes
nl_object_identical to default to comparing all attributes if
neither oo_id_attrs_get or oo_id_attrs are provided.

Signed-off-by: Thomas Graf <tgraf@suug.ch>