types: make the EvalType of TypeNull as ETInt

[XuHuaiyu]

What problem does this PR solve?

fix pingcap/tidb#16027

After pingcap/tidb#15512, TypeNull is defined as fixedLen. But the commit does not handle all the scenarios for TypeNull such https://github.com/pingcap/tidb/blob/master/expression/chunk_executor.go#L118.
We define the EvalType for TypeNull as EvalInt because the element length of TypeNull equals to TypeLonglong.

What is changed and how it works?

Define the EvalType of TypeNull as ETInt.

Check List

Tests

Test in tidb will cover this

Code changes

• Has exported function/method change

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #894 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #894      +/-   ##
==========================================
- Coverage   78.33%   78.30%   -0.03%     
==========================================
  Files          40       40              
  Lines       14745    14716      -29     
==========================================
- Hits        11550    11524      -26     
+ Misses       2508     2507       -1     
+ Partials      687      685       -2     

[XuHuaiyu]

Wrong modification.