[React 19] Using spread with binary conditional in JSX causes error in dev env for case of using legacy JSX transform

## Summary ### Prerequisites - React app which compiles with legacy JSX transform - Add a JSX element with spread operator with binary conditional, e.g.: ``` <button {...(isCountButtonActive && { onClick: () => setCount(currentCount => currentCount + 1) })} > Increment count </button> ``` - Run app in dev mode ### Result Error is thrown when conditional is evaluated to `false`: ``` Uncaught TypeError: Cannot use 'in' operator to search for '__self' in false at exports.createElement (chunk-Y64RAOT6.js?v=f4a3421f:775:86) at App (App.jsx:3:13) at callComponentInDEV (react-dom_client.js?v=f4a3421f:785:18) at renderWithHooks (react-dom_client.js?v=f4a3421f:3510:24) at updateFunctionComponent (react-dom_client.js?v=f4a3421f:4521:21) at beginWork (react-dom_client.js?v=f4a3421f:5096:20) at runWithFiberInDEV (react-dom_client.js?v=f4a3421f:864:18) at performUnitOfWork (react-dom_client.js?v=f4a3421f:7688:83) at workLoopSync (react-dom_client.js?v=f4a3421f:7580:43) at renderRootSync (react-dom_client.js?v=f4a3421f:7561:13) ``` Code fragment where it's thrown: ``` if (null != config) for (propName in didWarnAboutOldJSXRuntime || !("__self" in config) || "key" in config || (didWarnAboutOldJSXRuntime = true, warn("Your app (or one of its dependencies) is using an outdated JSX transform. Update to the modern JSX transform for faster performance: https://react.dev/link/new-jsx-transform")), hasValidRef(config), hasValidKey(config) && (checkKeyStringCoercion(config.key), typeString = "" + config.key), config) hasOwnProperty.call(config, propName) && "key" !== propName && "__self" !== propName && "__source" !== propName && (i[propName] = config[propName]); ``` ### Reproduction https://github.com/undeletable/jsx-spread-with-react-19 Click 'Toggle count button state' to reproduce