server: openSession instead set nil when changeUser (#33894)

[ti-srebot]

cherry-pick #33894 to release-5.3
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/35405

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

git push [email protected]:ti-srebot/tidb.git pr/35405:release-5.3-10461370d38e

---

What problem does this PR solve?

Issue Number:
close #33892
close #33699

Problem Summary:

What is changed and how it works?

Check List

Tests

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

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:

• XuHuaiyu
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

@knull-cn please accept the invitation then you can push to the cherry-pick pull requests. Comment with /cherry-pick-invite if the invitation is expired.
https://github.com/ti-srebot/tidb/invitations

[knull-cn]

/cherry-pick-invite

[knull-cn]

/cherry-pick-invite

[ti-srebot]

@knull-cn please accept the invitation then you can push to the cherry-pick pull requests. Comment with /cherry-pick-invite if the invitation is expired.
https://github.com/ti-srebot/tidb/invitations

[knull-cn]

/run-unit-test

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/db976a8f0fa8239efdf18fccaa4b4478b8fe2a33

[XuHuaiyu]

/merge

[ti-chi-bot]

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

Commit hash: 64c159f

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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