Undefined as a DefaultValue

It would be great, if we could write this: `property: fragment('pathToProp', { defaultValue: undefined })`
Sometimes it needs when a server sends incomplete data. And we should distinguish if an object/array is empty or undefined.

Locate the fragment(...) entry point and inspect how defaultValue is handled for empty objects, arrays, and missing values. Confirm the expected distinction described in the issue, then add coverage for defaultValue: undefined and verify that existing empty-object and empty-array behavior remains distinct.

Scritto dal modello di indicizzazione a partire dal testo della issue.