Fix fallback storage returning undefined for missing keys

[hamdyelbatal122]

Fixes #4866

What was the issue?

When localStorage is unavailable or throws an exception, $persist falls back to an in-memory Map storage (dummy).

When checking if a persisted key exists, storageHas() ran storage.getItem(key) !== null. Because Map.prototype.get() returns undefined (unlike localStorage.getItem() which returns null), storageHas() returned true for keys that were never set. This caused Alpine to attempt reading the value, receiving undefined, and overwriting the initial value passed to $persist().

Changes made

1. Updated the fallback dummy storage getItem handler to return null when a key is missing from the map, bringing it in line with the standard Storage.getItem() specification.
2. Updated storageHas() to check that the returned value is neither null nor undefined.

Verification

• Tested normal localStorage functionality — remains unchanged.
• Tested fallback memory storage — initial values are now preserved correctly when localStorage throws or is absent.