[react-transition-group] Support optional classNames prop on <CSSTransition/>

[ybiquitous]

Since v2.8.0, classNames prop has been optional.

See also:

• https://github.com/reactjs/react-transition-group/blob/v2.8.0/CHANGELOG.md
• Add support for empty classNames reactjs/react-transition-group#481

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[typescript-bot]

@ybiquitous Thank you for submitting this PR!

🔔 @LKay @Epskampie - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

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[typescript-bot]

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[typescript-bot]

I just published @types/react-transition-group@2.8.0 to npm.