fix(react): allow nullable controlled RadioGroup values

[raashish1601]

Fixes #3844.

Summary

• allow RadioGroup to accept null as the controlled empty-state value without widening the option type emitted by onChange
• extend useControllable so components can store a broader controlled value type than the narrower value type they emit
• add a compile-only regression fixture for value={selected ?? null} with onChange={setSelected}

Testing

• npx tsc -p packages/@headlessui-react/tsconfig.json --noEmit
• npx jest packages/@headlessui-react/src/components/radio-group/radio-group.test.tsx --runInBand
• npx prettier --check packages/@headlessui-react/src/components/radio-group/radio-group.tsx packages/@headlessui-react/src/hooks/use-controllable.ts packages/@headlessui-react/types/radio-group.test.tsx

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