fix(react): allow null controlled RadioGroup values

[raashish1601]

Summary

• allow RadioGroup to accept null as the controlled empty value again without broadening onChange to nullable values
• keep the controlled-empty path typed through useControllable so the selected value can stay null until an option is chosen
• add a regression that proves the null -> string controlled flow updates without triggering the uncontrolled-to-controlled warning

Testing

• npx prettier --check packages/@headlessui-react/src/hooks/use-controllable.ts packages/@headlessui-react/src/components/radio-group/radio-group.tsx packages/@headlessui-react/src/components/radio-group/radio-group.test.tsx
• npx jest packages/@headlessui-react/src/components/radio-group/radio-group.test.tsx --config packages/@headlessui-react/jest.config.cjs --runInBand
• npx tsc --pretty false --noEmit --jsx react-jsx --module esnext --moduleResolution node --target esnext --strict --esModuleInterop --skipLibCheck false ./.tmp-headlessui-3844-dist.tsx (run against the built @headlessui/react package with value={selected ?? null})
• ../../scripts/build.sh --external:react --external:react-dom --external:use-sync-external-store from packages/@headlessui-react under Git Bash with the workspace node_modules/.bin on PATH

Closes #3844.

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