TiFlash panics with `Data type Nullable(Nothing) cannot be used in tables` while syncing expression index schema `((null))`

[solotzg]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE `t` (
  `id` int,
  KEY `idx_name` ((null))
);
alter table t set tiflash replica 1;
insert t values(0);

2. What did you expect to see? (Required)

3. What did you see instead (Required)

tiflash panics;

error logs:

Code: 370, e.displayText() = DB::Exception: Data type Nullable(Nothing) cannot be used in tables

4. What is your TiFlash version? (Required)

>= v6.5.0, where expression index is GA