Port _nil objects to Null() machinery

Finally, unified!

src/hb-shape-plan-private.hh

diff --git a/src/hb-shape-plan-private.hh b/src/hb-shape-plan-private.hh
index c2c4987..7d020ff 100644
--- a/src/hb-shape-plan-private.hh
+++ b/src/hb-shape-plan-private.hh
@@ -51,6 +51,7 @@
 
   struct hb_shaper_data_t shaper_data;
 };
+DECLARE_NULL_INSTANCE (hb_shape_plan_t);
 
 #define HB_SHAPER_DATA_CREATE_FUNC_EXTRA_ARGS \
 	, const hb_feature_t *user_features \