expression: Fix the decimal fraction of DIV

[zz-jason]

drop table if exists t;
create table t(a bigint, b decimal(6, 2));
insert into t values(1, 1.12), (0, 1.12);

Before this PR:

TiDB(localhost:4000) > select a/b from t;
+--------+
| a/b    |
+--------+
| 0.8929 |
|      0 |
+--------+
2 rows in set (0.01 sec)

After This PR:

TiDB(localhost:4000) > select a/b from t;
+--------+
| a/b    |
+--------+
| 0.8929 |
| 0.0000 |
+--------+
2 rows in set (0.00 sec)

fix #6564

[zz-jason]

/run-all-tests

[iamxy]

/run-common-test

[iamxy]

/run-integration-common-test

[zz-jason]

/run-all-tests

[zz-jason]

/run-all-tests tidb-test=pr/536

[zz-jason]

/run-integration-common-test

[zz-jason]

@XuHuaiyu @winoros @tiancaiamao @zimulala PTAL

[zhexuany]

LGTM

[tiancaiamao]

LGTM