Unexpected result when using equal operator <=>

Consider the below queries, it is unexpected that two queries both return 0.

The Null-safe equal returns `1` if both operands are `NULL`. Thus the second query should return `1`, since `t0.c1` is `NULL`, and thus the expression `t0.c1<=>t0.c1 IS NULL` should be evaluated to `0`, and `NOT 0` is `1`.

DROP DATABASE db0;

CREATE DATABASE db0;

USE db0;

CREATE TABLE t0(c1 INT);

INSERT INTO t0 (c1) VALUES (NULL);

SELECT (t0.c1<=>t0.c1) IS NULL FROM t0; -- 0

SELECT NOT ((t0.c1<=>t0.c1) IS NULL) FROM t0; -- 0 (unexpected)