Fix errors in find and filter with an empty store

[snstanton]

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Description

In MockStore, this change guards against an empty store in calls to filter/find to avoid calling Object.values() on undefined.

Related #6262

Type of change

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• Bug fix (non-breaking change which fixes an issue)

Screenshots/Sandbox (if appropriate/relevant):

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How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can
reproduce. Please also list any relevant details for your test configuration

• store.spec.ts:filter method
• store.spec.ts:find method

Test Environment:

• OS: macos
• @graphql-tools/mock: 9.0.3
• NodeJS: v22.2.0

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

None

[changeset-bot]

🦋 Changeset detected

Latest commit: 0b95888

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphql-tools/mock	Patch

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[ardatan]

Thanks for the PR!
Could you create a changeset using yarn changeset then we can merge and release?