radicle/tor: Fixes — heartwood

Fixes two earlier mistakes:

In fb18083, fn null_to_default was added, which is only conditionally used if the feature “tor” is enabled, so also only conditionally compile the function.
In 1e13268, the impl Arbitrary for AddressType` was not properly adjusted for the case where the newly introduced feature “tor” is disabled.

 }
 /// Deserialize a value, but if it is `null`, return the default value.
 #[cfg(feature = "tor")]
 pub(crate) fn null_to_default<'de, D, T>(deserializer: D) -> Result<T, D::Error>
 where
     T: serde::Deserialize<'de> + Default,

 impl Arbitrary for AddressType {
     fn arbitrary(g: &mut qcheck::Gen) -> Self {
         let t = *g.choose(&[1, 2, 3, 4]).unwrap() as u8;
         #[cfg(not(feature = "tor"))]
         let types = [1, 2, 3];
         #[cfg(feature = "tor")]
         let types = [1, 2, 3, 4];
         let t = *g.choose(&types).unwrap() as u8;
         AddressType::try_from(t).unwrap()
     }