test: fix test for empty response with json media type

[wolfy1339]

Follow-up to #648

Resolves #ISSUE_NUMBER

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Before the change?

• The test written in fix: avoid Unexpected end of JSON input when response body is empty #648 didn't fully satisfy jest in terms of coverage, see the results of the coverage for refactor: use node: specifier imports and full relative paths imports, update tests for new type errors #653: https://github.com/octokit/request.js/actions/runs/7025892029/job/19117586525?pr=653#step:5:19
• The body option to fetch-mock was set to undefined

After the change?

• The test fully satisfies jest coverage
• Sets the body option to an empty string ""

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[wolfy1339]

It seems there is already a test in place but it doesn't seem to cover the function properly for coverage

[github-actions]

🎉 This PR is included in version 8.2.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 8.2.0 🎉

The release is available on:

• GitHub release
• npm package (@release-8.x dist-tag)

Your semantic-release bot 📦🚀