py: Use MP_OBJ_NULL instead of NULL in a few places.

py/objstr.c

@@ -215,7 +215,7 @@ STATIC mp_obj_t bytes_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_k
     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) {

py/objtype.c

@@ -292,7 +292,7 @@ mp_obj_t instance_make_new(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, c
     o = new_ret;
 
     // now call Python class __init__ function with all args
-    init_fn[0] = init_fn[1] = NULL;
+    init_fn[0] = init_fn[1] = MP_OBJ_NULL;
     lookup.obj = o;
     lookup.attr = MP_QSTR___init__;
     lookup.meth_offset = 0;

py/runtime.c

@@ -475,8 +475,8 @@ mp_obj_t mp_binary_op(mp_uint_t op, mp_obj_t lhs, mp_obj_t rhs) {
         }
         if (type->getiter != NULL) {
             /* second attempt, walk the iterator */
-            mp_obj_t next = NULL;
             mp_obj_t iter = mp_getiter(rhs);
+            mp_obj_t next;
             while ((next = mp_iternext(iter)) != MP_OBJ_STOP_ITERATION) {
                 if (mp_obj_equal(next, lhs)) {
                     return mp_const_true;