py: Use MP_OBJ_NULL instead of NULL when appropriate.
diff --git a/py/map.c b/py/map.c
index 6864513..087f320 100644
--- a/py/map.c
+++ b/py/map.c
@@ -306,7 +306,7 @@
         if (lookup_kind & MP_MAP_LOOKUP_ADD_IF_NOT_FOUND) {
             mp_set_rehash(set);
         } else {
-            return NULL;
+            return MP_OBJ_NULL;
         }
     }
     mp_uint_t hash = MP_OBJ_SMALL_INT_VALUE(mp_unary_op(MP_UNARY_OP_HASH, index));