executor: fix admin show job panic when upgrade from v4.0 to v6.4

[tangenta]

What problem does this PR solve?

Issue Number: close #38980

Problem Summary:

For the add index jobs from TiDB v4.0, it is possible to have job.ReorgMeta to be nil.

What is changed and how it works?

Check if job.ReorgMeta is nil before using it.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• crazycs520
• wjhuang2016

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[zimulala]

Do you have a test(Manual test) for this code?

[tangenta]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ccab286

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #39074.

[sre-bot]

TiDB MergeCI notify

✅ Well Done! New fixed [1] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🔴 failed 1, success 5, total 6	44 min	Existing failure
idc-jenkins-ci-tidb/mybatis-test	🔴 failed 1, success 0, total 1	11 min	Existing failure
idc-jenkins-ci/integration-cdc-test	✅ all 39 tests passed	21 min	Fixed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	8 min 56 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 46 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 45 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 22 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	3 min 8 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed