executor: fix admin show job panic when upgrade from v4.0 to v6.4 (#39067)

[ti-chi-bot]

This is an automated cherry-pick of #39067

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]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

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[wuhuizuo]

/run-check_dev_2

[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev_2	cdebc39	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	cdebc39	link	true	/test mysql-test

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