planner: fix potential panic when generating hints in statement summary (#46312)

[ti-chi-bot]

This is an automated cherry-pick of #46312

What problem does this PR solve?

Issue Number: close #46311

Problem Summary:
Use empty slices to init PlannerSelectBlockAsName to avoid panic when dereferencing.

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. create a cluster
2. run script like

func main() {
	cfg := &config{}
	cfg.parse()
	connStr := cfg.db + "?maxAllowedPacket=512000000&charset=utf8mb4&parseTime=True&loc=Local&allowAllFiles=true"
	var wg sync.WaitGroup
	for i := 0; i < 100; i++ {
		wg.Add(1)
		go func() {
			db, err := sql.Open("mysql", connStr)
			if err != nil {
				log.Fatal(err)
			}
			defer db.Close()

			stmt, err := db.Prepare("select t.i, t.j from t, t1 where t.k = 0")
			if err != nil {
				log.Fatal(err)
			}

			stmt1, err := db.Prepare("select t.i, t.j from t, t1 where t.i = 0")
			if err != nil {
				log.Fatal(err)
			}

			for {
				rs, err := stmt.Query()
				if err != nil {
					log.Println(err)
					break
				}
				for rs.Next() {
					var i int
					var j int
					err = rs.Scan(&i, &j)
					if err != nil {
						log.Println(err)
						break
					}
					log.Println(i, j)
				}
				rs.Close()

				rs1, err := stmt1.Query()
				if err != nil {
					log.Println(err)
					break
				}
				for rs1.Next() {
					var i int
					var j int
					err = rs1.Scan(&i, &j)
					if err != nil {
						log.Println(err)
						break
					}
					log.Println(i, j)
				}
				rs1.Close()
			}
			wg.Done()
		}()
	}
	wg.Wait()
}

3. check the panic log in tidb-server (I cannot reproduce the panic mentioned in the issue before PR.)

• 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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Defined2014, jackysp

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• OWNERS [Defined2014,jackysp]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-24 07:09:29.231127279 +0000 UTC m=+1392533.780143265: ☑️ agreed by jackysp.
• 2023-08-24 08:11:51.242425688 +0000 UTC m=+1396275.791441675: ☑️ agreed by Defined2014.

[Defined2014]

/test check-dev

[tiprow]

@Defined2014: No presubmit jobs available for pingcap/tidb@release-7.1

In response to this:

/test check-dev

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.1@b19b65c). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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