config: fix the panic caused by zero RegionSplitSizeMB

[JmPotato]

What problem does this PR solve?

Issue Number: close #8323.

What is changed and how does it work?

Bypass the case that `RegionSplitSizeMB` might be zero in `(*StoreConfig) CheckRegionSize`.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-06-25 09:14:21.310113839 +0000 UTC m=+710987.795602666: :ballot_box_with_check: agreed by nolouch.
• 2024-06-25 09:35:54.700305952 +0000 UTC m=+712281.185794785: :ballot_box_with_check: agreed by HuSharp.

[JmPotato]

/hold

[JmPotato]

/unhold

[ti-chi-bot[bot]]

@okJiang: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to [this](https://github.com/tikv/pd/pull/8324#pullrequestreview-2138059683): > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, niubell, nolouch, okJiang

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

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tikv/pd/blob/master/OWNERS)~~ [HuSharp,nolouch] - ~~[pkg/schedule/config/OWNERS](https://github.com/tikv/pd/blob/master/pkg/schedule/config/OWNERS)~~ [niubell] 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]

In response to a cherrypick label: new pull request created to branch release-8.1: #8325.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #8326.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #8327.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #8328.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #8329.