statistics: fix missing nil check for mv index

[time-and-fate]

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

Merging #50313 (ad5c56a) into master (e3abcbb) will decrease coverage by 14.6092%.
Report is 12 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #50313         +/-   ##
=================================================
- Coverage   70.0692%   55.4600%   -14.6092%     
=================================================
  Files          1443       1556        +113     
  Lines        419903     588095     +168192     
=================================================
+ Hits         294223     326158      +31935     
- Misses       105398     239209     +133811     
- Partials      20282      22728       +2446     

Flag	Coverage Δ
integration	36.6709% <100.0000%> (?)
unit	70.0599% <100.0000%> (-0.0094%)	⬇️

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

Components	Coverage Δ
dumpling	54.0269% <ø> (ø)
parser	∅ <ø> (∅)
br	55.6400% <ø> (+5.9160%)	⬆️

[elsa0520]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, elsa0520

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-11 08:16:09.219649345 +0000 UTC m=+517558.803903034: ☑️ agreed by AilinKid.
• 2024-01-11 08:24:07.557044197 +0000 UTC m=+518037.141297880: ☑️ agreed by elsa0520.

[hawkingrei]

/retest

[time-and-fate]

/test unit-test

[tiprow]

@time-and-fate: The specified target(s) for /test were not found.
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/test unit-test

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[time-and-fate]

/test check-dev2

[tiprow]

@time-and-fate: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

• /test tiprow_fast_test

Use /test all to run all jobs.

In response to this:

/test check-dev2

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[hawkingrei]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #50342.