statistics: fix missing nil check for mv index (#50313)

[ti-chi-bot]

This is an automated cherry-pick of #50313

What problem does this PR solve?

Issue Number: close #50298

Problem Summary:

Estimation logic for mv index access path will meet panic if it's a new index without stats.

What changed and how does it work?

Add a nil check.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.6@a0ce8e0). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.6     #50342   +/-   ##
================================================
  Coverage               ?   70.0117%           
================================================
  Files                  ?       1436           
  Lines                  ?     419107           
  Branches               ?          0           
================================================
  Hits                   ?     293424           
  Misses                 ?     105383           
  Partials               ?      20300           

Flag	Coverage Δ
unit	70.0117% <0.0000%> (?)

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

Components	Coverage Δ
dumpling	54.0269% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	49.6569% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, time-and-fate

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-11 11:58:25.563340484 +0000 UTC m=+530895.147594175: ☑️ agreed by AilinKid.
• 2024-01-11 12:59:09.57324993 +0000 UTC m=+534539.157503617: ☑️ agreed by time-and-fate.

[time-and-fate]

/test mysql-test

[tiprow]

@time-and-fate: No presubmit jobs available for pingcap/[email protected]

In response to this:

/test mysql-test

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