Function deserialize_nullable_patch_option 

pub fn deserialize_nullable_patch_option<'de, D, T>(
    deserializer: D,
) -> Result<Option<NullablePatch<T>>, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de>,

Deserializes an optional PATCH field while preserving explicit null.

Use this with #[serde(default, deserialize_with = "...")] on Option<NullablePatch<T>> fields when the surrounding DTO needs to distinguish omitted fields from explicit nulls.