chunk: make mysql.TypeNull be judged as fixedLen

[lysu]

What problem does this PR solve?

Issue Number: close #6252

Problem Summary:

What is changed and how it works?

fix:

MySQL [test]> create table t2(a int);
Query OK, 0 rows affected (0.013 sec)
MySQL [test]> create table t1(a int);
Query OK, 0 rows affected (0.011 sec)
MySQL [test]> insert into t1(a) select 1;
Query OK, 1 row affected (0.004 sec)
Records: 1 Duplicates: 0 Warnings: 0
MySQL [test]> select b.n from t1 left join (select a as a, null as n from t2) b on b.a = t1.a order by t1.a;
ERROR 1105 (HY000): runtime error: index out of range [1] with length 1

What's Changed:

try let TypeNull as fixedLen column in the chunk

How it Works:

try let TypeNull as fixedLen column in the chunk

Related changes

• Need to cherry-pick to the release branch: same problem in 2.1/3.0

Check List

Tests

WIP

• Unit test
• Integration test

Side effects

• n/a

Release note

fix panic when sort/topn above left join with null column in right child

---

This change is 

[lysu]

bug caused by

tidb/executor/joiner.go

Line 713 in c1114b9

chk.AppendPartialRowByColIdxs(lWide, j.defaultInner, j.rUsed)

source chunk j.defaultInner is fixedLen null column, but destination chunk chk is varLen null column.

[codecov]

Codecov Report

Merging #15512 into master will decrease coverage by 0.0685%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             master     #15512        +/-   ##
================================================
- Coverage   80.5692%   80.5006%   -0.0686%     
================================================
  Files           504        502         -2     
  Lines        135203     134348       -855     
================================================
- Hits         108932     108151       -781     
+ Misses        17774      17741        -33     
+ Partials       8497       8456        -41

[lysu]

/run-all-tests

[lysu]

/run-unit-test

[qw4990]

LGTM

[zz-jason]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for 15628, 15635, 15711

[sre-bot]

/run-all-tests

[sre-bot]

cherry pick to release-3.0 in PR #15738

[sre-bot]

cherry pick to release-3.1 in PR #15739

[sre-bot]

cherry pick to release-4.0 in PR #15740

[lysu]

/run-cherry-picker

[sre-bot]

cherry pick to release-2.1 in PR #15930