Function null_as_default 

pub fn null_as_default<'de, T, D>(
    deserializer: D,
) -> Result<T, <D as Deserializer<'de>>::Error>
where
    T: Deserialize<'de> + Default,
    D: Deserializer<'de>,

For use with serde’s deserialize_with on a sequence that must be deserialized as a single but optional (i.e. possibly null) value.