runaway: only check statements with a non-empty plan digest

[nolouch]

What problem does this PR solve?

Issue Number: close https://github.com/pingcap/tidb/issues/56897

Problem Summary:

What changed and how does it work?

Check List

Tests

• [x] 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.

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: glorv, JmPotato

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](https://github.com/pingcap/tidb/blob/master/OWNERS)~~ [glorv] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-11-21 08:07:33.566275505 +0000 UTC m=+105441.185930020: :ballot_box_with_check: agreed by glorv.
• 2024-11-21 08:08:53.450112511 +0000 UTC m=+105521.069767024: :ballot_box_with_check: agreed by JmPotato.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.4067%. Comparing base (14e99ea) to head (e4d64b5). Report is 25 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #57582 +/- ## ================================================ + Coverage 72.8115% 73.4067% +0.5951% ================================================ Files 1676 1676 Lines 463414 468825 +5411 ================================================ + Hits 337419 344149 +6730 + Misses 105180 103802 -1378 - Partials 20815 20874 +59 ``` | [Flag](https://app.codecov.io/gh/pingcap/tidb/pull/57582/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/pingcap/tidb/pull/57582/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `43.4516% <13.3333%> (?)` | | | [unit](https://app.codecov.io/gh/pingcap/tidb/pull/57582/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `72.2040% <90.0000%> (+0.0004%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap#carryforward-flags-in-the-pull-request-comment) to find out more. | [Components](https://app.codecov.io/gh/pingcap/tidb/pull/57582/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [dumpling](https://app.codecov.io/gh/pingcap/tidb/pull/57582/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `52.7673% <ø> (ø)` | | | [parser](https://app.codecov.io/gh/pingcap/tidb/pull/57582/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `∅ <ø> (∅)` | | | [br](https://app.codecov.io/gh/pingcap/tidb/pull/57582/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `45.2453% <ø> (-0.0106%)` | :arrow_down: |

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #57599.