copr: avoid NPE when store is not available when balance batch cop task

[windtalker]

What problem does this PR solve?

Issue Number: close #28096

Problem Summary:
As described in the issue.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:
The root cause is in L162, it tries to get err.Error() even if err is nil

tidb/store/copr/batch_coprocessor.go

Lines 161 to 167 in b839744

	if err != nil || !resp.Resp.(*mpp.IsAliveResponse).Available {
	logutil.BgLogger().Warn("Cannot detect store's availability", zap.String("store address", s.GetAddr()), zap.String("err message", err.Error()))
	mu.Lock()
	mppStoreLastFailTime[s.GetAddr()] = time.Now()
	mu.Unlock()
	return
	}

The pr fix it by only get err.Error() if err is not nil.

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

Fix a bug that TiDB may crash when TiFlash is shutting down.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• hanfei1991

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[windtalker]

/label needs-cherry-pick-5.2

[windtalker]

/label needs-cherry-pick-5.1

[windtalker]

/label needs-cherry-pick-5.0

[fzhedu]

/LGTM

[fzhedu]

/merge

[ti-chi-bot]

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

Commit hash: 50174ec

[ti-srebot]

cherry pick to release-5.0 in PR #28138

[ti-srebot]

cherry pick to release-5.1 in PR #28139

[ti-srebot]

cherry pick to release-5.2 in PR #28140