fix(diffs): empty files marked as deleted

[AnatoleLucet]

Description

Hello 👋

I noticed a minor inconsistency while using newFile/oldFile between 1) rendering a diff of a file with content that didn't change, and 2) rendering a diff of an empty file that didn't change.

Motivation & Context

Current behavior:

File with content and no change

const fileDiffInstance = new FileDiff({ theme: "pierre-dark" });

fileDiffInstance.render({
  oldFile: { name: "my-file", contents: "abc" },
  newFile: { name: "my-file", contents: "abc" }, // content didn't change
  containerWrapper: document.body,
});

Empty file with no change

const fileDiffInstance = new FileDiff({ theme: "pierre-dark" });

fileDiffInstance.render({
  oldFile: { name: "my-file", contents: "" },
  newFile: { name: "my-file", contents: "" }, // stays empty
  containerWrapper: document.body,
});

We should expect both diff to be of type change, but the second one is rendered as deleted (even though the file has not been deleted, it's just empty).

New behavior:

File with content and no change

// same code as before and same result
const fileDiffInstance = new FileDiff({ theme: "pierre-dark" });

fileDiffInstance.render({
  oldFile: { name: "my-file", contents: "abc" },
  newFile: { name: "my-file", contents: "abc" },
  containerWrapper: document.body,
});

Empty file with no change

// same code as before, but now the rendered diff is "change", not "deleted"
const fileDiffInstance = new FileDiff({ theme: "pierre-dark" });

fileDiffInstance.render({
  oldFile: { name: "my-file", contents: "" },
  newFile: { name: "my-file", contents: "" },
  containerWrapper: document.body,
});

Now the empty file is correctly rendered as change and not deleted.

Type of changes

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

Checklist

• I have read the
  contributing guidelines
• My code follows the code style of the project (bun run lint)
• My code is formatted properly (bun run format)
• I have updated the documentation accordingly (if applicable)
• I have added tests to cover my changes (if applicable)
• All new and existing tests pass (bun run diffs:test)

How was AI used in generating this PR

No AI used at all. I just crawled trough the codebase like a caveman.

Related issues

#399-ish

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[amadeus]

Thanks for the fix! I probably will want to strengthen this a bit more in the future even (by using undefined-ness to make this more explicit, but for now I think this is a good change!