fix(form): prevent TypeError when form contains elements without validity

[sonsu-lee]

Closes #9424

✅ 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:

yarn test packages/react-aria-components/test/Form.test.js

🧢 Your Project:

[reidbarber]

LGTM, thanks for the fix and test!