TreeView: Add indication of empty directory

[TylerJDev]

Closes https://github.com/github/primer/issues/3447

Example story: Empty Directory

Adds indication of no nodes in a empty directory. Previously, the directory wouldn't indicate the user if the directory was empty or not, even though the icon changed to open/closed when interacting with it.

Example:

Screen reader support

	VoiceOver (Safari)	VoiceOver (Chrome)	NVDA (Firefox)
Announces "empty" live region	Yes	Yes	Yes
Announces empty node	Yes	Yes	Yes
Announces expanded/collapsed state	Yes	Yes	Yes

Changelog

New

• Adds indication of no nodes in tree item/directory via EmptyItem

Changed

• Allows for aria-expanded even if tree item is empty

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 5ace102

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	98.39 KB (+0.04% 🔺)
packages/react/dist/browser.umd.js	98.75 KB (+0.05% 🔺)

[primer-integration]

🟢 golden-jobs completed with status success.

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/349856