Function nullable_value 

pub fn nullable_value<'de, D>(
    deserializer: D,
) -> Result<Option<Box<Value>>, D::Error>
where
    D: Deserializer<'de>,

Custom deserializer that distinguishes “field absent” from “field: null”.

• JSON field absent → None (via #[serde(default)])
• JSON field null → Some(Value::Null)
• JSON field with value → Some(value)

Use with #[serde(default, skip_serializing_if = "Option::is_none", deserialize_with = "nullable_value")]