React version: 18.0.0-beta-4ff5f5719-20211115

Steps To Reproduce

1. Render empty string ReactDOMServer.renderToString("")

Link to code example: https://codesandbox.io/s/react-18-emptry-string-hydration-mismatch-xz4w9
Original issue: f9d729e (#1541)

Did some spelunking in the codebase and it seems to me that the reconciler is looking for a hydrateable instance (https://github.com/facebook/react/blob/00ced1e2b7610543a519329a76ad0bfd12cd1c32/packages/react-reconciler/src/ReactFiberBeginWork.new.js#L1413-L1415). But since an empty string won't appear as a text node (if we just set innerHTML = string), the reconciler thinks there's a mismatch. In legacy roots we didn't throw but warn unless we had an empty string: https://github.com/facebook/react/blob/75f3ddebfa0d9885ce8df42571cf0c09ad6c0a3b/packages/react-dom/src/client/ReactDOMComponent.js#L1202-L1208

The current behavior

Console error is logged with "An error occurred during hydration. The server HTML was replaced with client content"

The expected behavior

No hydration mismatch just like in React 17: https://codesandbox.io/s/react-17-emptry-string-no-hydration-mismatch-forked-5tgmw