executor: fix stmt summary nil pointer panic on the cached table

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #31077

Problem Summary:

It's a bug introduced in https://github.com/pingcap/tidb/pull/30166/files#diff-cda21674d674295dde958bfdfe5d6ecd21ae07fd310fa907920e1214108d974fR1198

When there isn't any coprocessor request, the map is nil, so set the nil map cause panic.

The old code comes with test, but the test doesn't really cover that line,
that's because there is a special judgement here, when AuthUser string is empty, the stmt summary code is skipped!

tidb/executor/adapter.go

Line 1163 in 57b2908

if !stmtsummary.StmtSummaryByDigestMap.Enabled() || ((sessVars.InRestrictedSQL || len(userString) == 0) && !stmtsummary.StmtSummaryByDigestMap.EnabledInternal()) {

What is changed and how it works?

Fix the nil pointer panic.

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lcwangchao
• xhebox

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/9de73b23f09fcacd33e4de0b982819a05fd9a956

[tiancaiamao]

/run-check_dev_2

[tiancaiamao]

/run-check_dev_2

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 8232448

[ti-chi-bot]

@tiancaiamao: Your PR was out of date, I have automatically updated it for you.

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[tiancaiamao]

/run-unit-test