Function null_default 

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

Decode null as the type’s default rather than as a failure.

Applied to every composite field — see the module docs for why a null arrives at all, and why scalars deliberately do not get this treatment.

Errors

Propagates the underlying decode failure for anything that is neither null nor a valid value of the field’s type.