chunk: make mysql.TypeNull be judged as fixedLen (#15512)

[sre-bot]

cherry-pick #15512 to release-3.1

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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

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This change is 

[sre-bot]

/run-all-tests

[zz-jason]

@lysu please resolve conflicts

[lysu]

/run-all-tests

[zimulala]

LGTM

[zz-jason]

LGTM

[sre-bot]

/run-all-tests