fix(#5747): [react-aira: RadioGroup]: No tabindex/Can't keyboard focu…

…s to input until value is checked (#5755)

Looks like the default selected value in the example is an empty string, which is not the same as null. https://github.com/adobe/react-spectrum/pull/4985/files#diff-26ddea7c3bd0c2b48179af98635706a6391a01b59c936e8b497d1843a97ff202R71-R78

A more robust solution might be to validate whether the default value is one of the possible values within the radio group, but that's a bit more complicated.

Co-authored-by: Robert Snow <rsnow@adobe.com>
Co-authored-by: Daniel Lu <dl1644@gmail.com>

packages/react-aria-components/docs/RadioGroup.mdx

@@ -261,15 +261,15 @@ The `onChange` event is fired when the user selects a radio.
 
 ```tsx example
 function Example() {
-  let [selected, setSelected] = React.useState('');
+  let [selected, setSelected] = React.useState(null);
 
   return (
     <>
       <MyRadioGroup label="Favorite avatar" value={selected} onChange={setSelected}>
         <Radio value="wizard">Wizard</Radio>
         <Radio value="dragon">Dragon</Radio>
       </MyRadioGroup>
-      <p>You have selected: {selected}</p>
+      <p>You have selected: {selected ?? ''}</p>
     </>
   );
 }