expr: fix tikv crash when conv empty string (#12672)

[ti-srebot]

cherry-pick #12672 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tikv repo:
git pr https://github.com/tikv/tikv/pull/12688

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

git push git@github.com:ti-srebot/tikv.git pr/12688:release-4.0-62d77776088f

---

What is changed and how it works?

Issue Number: Close #12673

What's Changed:

Fix tikv crash when conv empty string

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

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

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

Fix tikv crash when conv empty string

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• youjiali1995

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[ti-chi-bot]

@ti-srebot: 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.

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[ti-srebot]

/run-all-tests

[ti-srebot]

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