Add null type to Radiogroup value prop. (#5896)

* add null type to RadioGroup.props.value

* add a controlled RadioGroup story

---------

Co-authored-by: Reid Barber <reid@reidbarber.com>
Co-authored-by: Yihui Liao <44729383+yihuiliao@users.noreply.github.com>

Author: 3 people

packages/@react-types/radio/src/index.d.ts

@@ -29,7 +29,7 @@ import {
 } from '@react-types/shared';
 import {ReactElement, ReactNode} from 'react';
 
-export interface RadioGroupProps extends ValueBase<string>, InputBase, InputDOMProps, Validation<string | null>, LabelableProps, HelpTextProps, FocusEvents {
+export interface RadioGroupProps extends ValueBase<string|null, string>, InputBase, InputDOMProps, Validation<string | null>, LabelableProps, HelpTextProps, FocusEvents {
   /**
    * The axis the Radio Button(s) should align with.
    * @default 'vertical'

packages/react-aria-components/stories/RadioGroup.stories.tsx

@@ -11,7 +11,7 @@
  */
 
 import {Button, Dialog, DialogTrigger, Label, Modal, ModalOverlay, Radio, RadioGroup} from 'react-aria-components';
-import React from 'react';
+import React, {useState} from 'react';
 import styles from '../example/index.css';
 
 export default {
@@ -31,6 +31,23 @@ export const RadioGroupExample = () => {
   );
 };
 
+export const RadioGroupControlledExample = () => {
+  let [selected, setSelected] = useState<string|null>(null);
+  
+  return (
+    <RadioGroup
+      data-testid="radio-group-example"
+      className={styles.radiogroup}
+      value={selected}
+      onChange={setSelected}>
+      <Label>Favorite pet (controlled)</Label>
+      <Radio className={styles.radio} value="dogs" data-testid="radio-dog">Dog</Radio>
+      <Radio className={styles.radio} value="cats">Cat</Radio>
+      <Radio className={styles.radio} value="dragon">Dragon</Radio>
+    </RadioGroup>
+  );
+};
+
 export const RadioGroupInDialogExample = () => {
   return (
     <DialogTrigger>