fix(react): skip undefined props in attachProps (v9)

[ptmkenny]

Issue number: resolves #31344

This is the v9 version of the PR #31345

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[ShaneK]

Hey @ptmkenny! Sorry to do this twice, but I'm closing this one too and going with a different approach in #31361

Skipping the assignment for nullish values changes what undefined means for a prop that has a default. Right now <IonModal backdropDismiss={maybeUndefined} /> writes undefined over the @Prop() default of true, so with the assignment skipped the backdrop would start dismissing when it didn't before. It would also leave the generated components behaving differently, since those go through @lit/react, which assigns and then removes the attribute.

The new one mirrors @lit/react instead, and it has the tests you asked for. Thanks for the reproduction, it made this a lot easier to work through.