fix(wallet): don't fail reading Null change_desc column from sqlite

If no change descriptor is persisted then `Row::get` fails with
`InvalidColumnType`. This is fixed by changing the FromSql type
to `Option<Impl<Descriptor<DescriptorPublicKey>>>`.

crates/wallet/src/wallet/changeset.rs

@@ -104,14 +104,23 @@ impl ChangeSet {
             .query_row([], |row| {
                 Ok((
                     row.get::<_, Impl<Descriptor<DescriptorPublicKey>>>("descriptor")?,
-                    row.get::<_, Impl<Descriptor<DescriptorPublicKey>>>("change_descriptor")?,
+                    row.get::<_, Impl<Descriptor<DescriptorPublicKey>>>("change_descriptor"),
                     row.get::<_, Impl<bitcoin::Network>>("network")?,
                 ))
             })
             .optional()?;
-        if let Some((Impl(desc), Impl(change_desc), Impl(network))) = row {
+        if let Some((Impl(desc), res, Impl(network))) = row {
             changeset.descriptor = Some(desc);
-            changeset.change_descriptor = Some(change_desc);
+            use crate::rusqlite::types::Type;
+            use crate::rusqlite::Error::*;
+            changeset.change_descriptor = match res {
+                Ok(Impl(change_desc)) => Some(change_desc),
+                Err(e) => match e {
+                    // allow a Null column for change descriptor
+                    InvalidColumnType(_, _, Type::Null) => None,
+                    _ => return Err(e),
+                },
+            };
             changeset.network = Some(network);
         }