executor: do not build range for NullOuterVal in IndexLookUpJoin

[XuHuaiyu]

What problem does this PR solve?

fix #8499

Before this PR, the result of the following query is wrong.

CREATE TABLE `t1` (
  `a` int(11) NOT NULL,
  `b` int(11) DEFAULT NULL,
  PRIMARY KEY (`a`)
);
insert into t1 values(1, 0), (2, null);
CREATE TABLE `t2` (
  `a` int(11) NOT NULL,
  PRIMARY KEY (`a`)
);
insert into t2 values(0);
tidb> select /*+ TIDB_INLJ(t2)*/ * from t1 left join t2 on t1.b = t2.a;
+---+------+------+
| a | b    | a    |
+---+------+------+
| 1 |    0 |    0 |
| 1 |    0 |    0 |
| 2 | NULL | NULL |
+---+------+------+
3 rows in set (0.00 sec)

What is changed and how it works?

When the innerPlan is a TableReader, the range of PKHanle will be built in buildTableReaderForIndexJoin,
and NilDatum.GetInt64() will return 0. Thus we read the row with PkHandle=0 twice, and get a duplicate result line.

When the innerPlan is a IndexReader or IndexLookUpReader, we'll encode the NilDatum, thus no error happens.

Check List

Tests

• Integration test

Code changes

• Has exported function/method change

Side effects
none

Related changes

• Need to cherry-pick to the release branch

---

This change is 

[XuHuaiyu]

/run-all-tests

[XuHuaiyu]

PTAL @zz-jason @winoros @eurekaka @lamxTyler

[zz-jason]

LGTM

[zz-jason]

@XuHuaiyu we should cherry pick this change to release-2.1 and release-2.0

[eurekaka]

LGTM

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[XuHuaiyu]

/run-integration-ddl-test