Add null type to Radiogroup value prop.

[sookmax]

Closes #5871

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Story

Controlled RadioGroup

http://localhost:9003/?path=/story/react-aria-components--radio-group-controlled-example&providerSwitcher-express=false&strict=true

🧢 Your Project:

[LFDanLu]

LGTM, thanks!

[sookmax]

@LFDanLu Hi, I have a question that's unrelated to this PR.

does every PR deploy the associated storybook somewhere? I think I saw some long domain names that include microsoft or windows somewhere in other people's PRs.. that looked like a storybook instance.. but I'm not sure 😅.

The reason why I ask this is that I thought it would be nicer to share the deployed storybook for a PR (if exists) than using localhost:9003 😭.

[LFDanLu]

@sookmax For contributor PRs we don't automatically run the storybook build for security reasons hence why you might notice the occasional comment from a team member of "GET_BUILD" that triggers it manually.

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

## API Changes

unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any', access: 'private' }
unknown top level export { type: 'any', access: 'private' }
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unknown top level export { type: 'identifier', name: 'Column' }
unknown top level export { type: 'identifier', name: 'Column' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown type { type: 'link' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }
unknown top level export { type: 'any' }