Unexpected result of function LOCATE when one of arguments is NULL

Hi,

Based on the document https://mariadb.com/docs/server/reference/sql-functions/string-functions/locate, if any argument of LOCATE is NULL, LOCATE returns NULL.

However, it returns 0 in the following test case:

```
CREATE TABLE t0(c0 REAL);
CREATE TABLE IF NOT EXISTS t1 LIKE t0;
INSERT INTO t0 VALUES (-1);
SELECT t1.c0, LOCATE(t0.c0, t0.c0, t1.c0) FROM t0 LEFT JOIN t1 ON false;

c0 LOCATE(t0.c0, t0.c0, t1.c0)
NULL 0
```
I tried this in MySQL and found it returns NULL.