Review nullability of DoclingDocument

[ThomasVitale]

• Based on the Docling API, ensure each nullable field is marked as @Nullable.
• For collections, adopt the same pattern used in Docling Serve and Docling Client so that null collections are deserialised into empty collections.
• For boolean, use base type instead of wrapper to ensure a false default as in the Docling Core API project.
• For numerical properties, keep using wrappers as the defaults might change.