Core upon modifing char to varchar

```
CREATE TABLE `aggregate_table_with_null_2` (
  `k1` date NULL COMMENT "",
  `add_key` boolean REPLACE NULL COMMENT ""
) ENGINE=OLAP
AGGREGATE KEY(`k1`)
COMMENT "OLAP"
DISTRIBUTED BY HASH(`k1`) BUCKETS 3
PROPERTIES (
"replication_num" = "1",
"in_memory" = "false",
"storage_format" = "V2"
);
```