ddl: tidb panic while query hash partition table with is null condition (#23849)

[ti-srebot]

cherry-pick #23849 to release-5.0
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/26963

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

git push [email protected]:ti-srebot/tidb.git pr/26963:release-5.0-5005bc08cb33

close #26779

---

What problem does this PR solve?

Issue Number: close #23802

Check List

Tests

• Unit test

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@qw4990 you're already a collaborator in bot's repo.

[time-and-fate]

/run-check-dev

[time-and-fate]

/run-check_dev

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: a2db40c

[ti-chi-bot]

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