*: fix wrong modification for introduced from #33818

[lcwangchao]

What problem does this PR solve?

Issue Number: close #33814

Problem Summary:

#33818 wrongly removed prepared.CachedPlan == nil in if statement, recover it

What is changed and 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

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:

• JmPotato
• Yisaer

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[lcwangchao]

/merge

[ti-chi-bot]

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

Commit hash: f38ada1

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/f38ada101f489acd240bc2ae438c9453c8b67f38

[lcwangchao]

/run-check_dev_2