fix(JSXElement): React19 regression: element._store may not be present in mixed dev/prod environment

[smeng9]

Summary

@yungsters @javache

Fix comments in #29662 (comment)

Hi @yungsters, we have encountered issue that such keys does not exist.

Our use case is as follows, we use microfrontend where some part of code is built with production flag and loaded through https://github.com/Paciolan/remote-component and some other part of the code is used in development mode.

Production code in react does not have private variables like _store and when cloning it certainly has problem.

Please check for the existences of _store.

How did you test this change?

React 18 does not have such issue, and upgrading to react 19 should behave the same when doing development.

[yungsters]

I submitted #32117, which also includes a unit test.