Fix RadioGroup to support zero as a Radio value

[stokesman]

There's an oversight in the RadioGroup component that makes it fail to allow a Radio with a value of 0 to be checked. Since the component does not appear to be used anywhere I've updated one of its Storybook stories to demonstrate the use case.

Screen recordings

Before fails to “check” the Radio with the zero value.

radio-group-zero-index.mp4

After

radio-group-zero-index-after.mp4

How has this been tested?

In a custom block using WP 5.6 and in Storybook

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[youknowriad]

LGTM 👍