test: add a failing test rendering null on client with SSR

[koba04]

Summary

This doesn't include a fix; it just adds a failing test.
The test shows React renders duplicate content when hydrateRoot renders null and then updates the actual content.
This is the hydration mismatch error, a common mistake in a loading pattern, so I think React should warn it or fix the duplicate content.
Do you have any idea which way I should take? I'll work on fixing this in the way we agree.

How did you test this change?

yarn test packages/react-dom/src/__tests__/ReactDOMHydrationDiff-test.js

[react-sizebot]

Comparing: e1d20fc...ed6af9f

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	176.24 kB	176.24 kB	=	54.88 kB	54.88 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	178.34 kB	178.34 kB	=	55.52 kB	55.53 kB
facebook-www/ReactDOM-prod.classic.js	=	571.94 kB	571.94 kB	=	100.56 kB	100.56 kB
facebook-www/ReactDOM-prod.modern.js	=	555.70 kB	555.70 kB	=	97.62 kB	97.62 kB
test_utils/ReactAllWarnings.js	Deleted	67.55 kB	0.00 kB	Deleted	16.54 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	67.55 kB	0.00 kB	Deleted	16.54 kB	0.00 kB

Generated by 🚫 dangerJS against ed6af9f

[github-actions]

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