fix: exclude null type from RadioGroup validate value

[lixiaoyan]

Closes

We will never receive the null value in the validate function. See https://github.com/adobe/react-spectrum/blob/ef3ff3d5d70302ad365a7c55bd71330967f3507e/packages/%40react-stately/form/src/useFormValidationState.ts#L93

✅ 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

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