This is an automated cherry-pick of #10162

What problem does this PR solve?

Issue Number: close #9891

Problem Summary:

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

If the user create an expression index as above, TiDB will generate a "column" for the expression with "Tp": 6. This type of column will generate ColumnNothing which is not expected to be created at the IStorage, thus causing exception thrown during the DDL syncing when TiFlash handling the RaftLog from the table.
Because TiFlash-proxy has persisted the RaftLog but get exception when decoding the RaftLog and apply it to the Storage layer, TiFlash panic.

What is changed and how it works?

ddl: Fix TiFlash panic when meets expression index with format `((NULL))`
Using `ColumnNothing` in IStorage may bring unexpected behavior when we try to write or read the column. So TiFlash create `ColumnInt8` instead of `ColumnNothing` when `TiDB::ColumnInfo.Tp == Null`.

However, we don't expected the column is used when reading or writing to TiFlash.

* `((null))` is not a normal expression index, this could be created by the user in accident (e.g. executing wrong SQL statement to create the expression index), TiDB should not write or read from it
* Even for normal expression index, TiDB does not use expression when accessing to TiFlash

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix the issue that TiFlash panic when meets expression index with format `((NULL))`