[LiveComponent] Fix (de)hydration of DTO collections containing a null item - #4194

When a LiveProp holds DTO objects in an array and one item is null, dehydrating the component throws:

LogicException: The LiveProp "..." on component "..." is an array. We determined the array is full of "..." objects, but at least one key had a different value of "null".

That null item is the normal state after LiveCollectionTrait::addCollectionItem() adds a row whose form entry has not been filled in yet, so adding a row to a DTO-backed LiveCollectionType breaks the component as soon as the collection already holds at least one record.

LiveComponentHydrator::dehydrateValue() rejected every item that did not match the collection's object type. hydrateValue() had the mirror problem: it passed null items into hydrateObjectValue(), which cannot hydrate them.

This keeps null items as null in both directions. The added integration test runs a CustomerDetails[] prop holding one filled DTO and one null item through a full dehydrate/hydrate round trip.