py: Use MP_OBJ_NULL instead of NULL in a few places.
diff --git a/py/objstr.c b/py/objstr.c
index 7e6fd10..b008dc6 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -215,7 +215,7 @@
     mp_int_t len;
     byte *data;
     vstr_t *vstr = NULL;
-    mp_obj_t o = NULL;
+    mp_obj_t o = MP_OBJ_NULL;
     // Try to create array of exact len if initializer len is known
     mp_obj_t len_in = mp_obj_len_maybe(args[0]);
     if (len_in == MP_OBJ_NULL) {