… writing (apache#41737)

CREATE TABLE t (
    `k1` VARCHAR(30) NOT NULL,
    `v1` INT NOT NULL
)

alter table t modify column `v1` INT NULL

insert into value ('1', 2), ('1', 3);

core dump

Schema change leads to double writing, during double writing, the two
schemas and slots are as follows

```
old tablet schema
k1 varchar not null
v1 int not null
```

```
new tablet scheam
k1 varchar not null
v1 int null
```

```
slot
k1 varchar not null
v1 int not null
v1 int null
```

During the double writing process, when selecting slots through the
schema, only the column names and types were compared, without comparing
the nullable attributes, which led to the selection of the wrong slot.
Since the slot determines the nullable attribute of the block, the
nullable attribute of the columns in the block is different from that of
the columns in the schema, resulting in a core dump.

…-alter-null-2.0