expression, executor: fix type infer for greatest/leastest(datetime) (#26533)

[ti-srebot]

cherry-pick #26533 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26564

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/26564:release-4.0-dd2fadb19085

---

What problem does this PR solve?

Issue Number: close #26532

Problem Summary:

What is changed and how it works?

What's Changed:
Make the flen of greatest/leatest(datetime) correct

How it Works:

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

• Fix the issue that greatest(datetime) union null returns empty string

[ti-srebot]

/run-all-tests

[ti-srebot]

@XuHuaiyu you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• guo-shaoge
• wshwsh12

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[wshwsh12]

/merge

[ti-chi-bot]

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

Commit hash: 25580a0

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