Function serialize_opt_angle 

pub fn serialize_opt_angle<S, T>(
    value: &Option<T>,
    serializer: S,
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>
where
    S: Serializer,
    T: Serialize,
    &'a T: for<'a> ToString,

Like serialize_angle, but serializes an [&Option<T>] instead of a &T implementing Into<DynQuantity>.

For examples see the serialize_with_units documentation.