planner: fix potential panic when generating hints in statement summary

[jackysp]

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

[jackysp]

/run-all-tests

[codecov]

Codecov Report

Merging #46312 (d31289b) into master (dd593b2) will decrease coverage by 0.2634%.
Report is 26 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #46312        +/-   ##
================================================
- Coverage   73.3535%   73.0902%   -0.2634%     
================================================
  Files          1284       1314        +30     
  Lines        393272     405247     +11975     
================================================
+ Hits         288479     296196      +7717     
- Misses        86425      90505      +4080     
- Partials      18368      18546       +178     

Flag	Coverage Δ
integration	26.0501% <100.0000%> (?)
unit	73.7880% <100.0000%> (+0.4345%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	54.0444% <ø> (ø)
parser	85.0850% <ø> (+0.0182%)	⬆️
br	48.0667% <ø> (-4.1817%)	⬇️

[jackysp]

/retest

[hawkingrei]

/retest

[jackysp]

/check-issue-triage-complete

[jackysp]

/check-issue-triage-complete

[jackysp]

/run-check-issue-triage-complete

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Defined2014,hawkingrei]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-24 00:42:42.183131577 +0000 UTC m=+1369326.732147549: ☑️ agreed by hawkingrei.
• 2023-08-24 02:00:55.592762268 +0000 UTC m=+1374020.141778255: ☑️ agreed by Defined2014.

[jackysp]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #46393.