Prevent unwanted persistence of 'undefined' in storage

[JackWH]

I noticed an issue in Filament which caused Alpine to crash due to a component inadvertently setting undefined as a value with $persist. Whilst debugging I found a recent PR #4091, which attempts to prevent a similar issue when retrieving an undefined value from storage.

However it seems users can still unintentionally persist an undefined value, which confusingly gets stringified to remain undefined before then being saved as a string regardless by localStorage setItem.

This PR prevents committing undefined to storage by swapping it out with null, which is valid JSON and can therefore be stringified. Future checks with storageHas will return false (as expected), falling back to the initial value. It might be worth raising a warning but I'll take any feedback on this approach first. Thanks!

[calebporzio]

Thanks @JackWH - can you provide a simple code snippet to demonstrate this problem? or better include a test for this?

[calebporzio]

Closing because of lack of response. Please re-submit if new info is presented. Thanks!