rust-config: show default `null` is coerced incorrectly to `false`

Probably being too trigger happy about boolean values, I incorrectly set
the transform for a `None` to a `Some(false)`. It would cause for example
the `ui.formatted` value to be set to `Some(false)`, which turns off the colors
among other things, when `None` would trigger the automatic behavior.

This is fixed in the next commit.

rust/hg-core/src/config/mod.rs

@@ -803,5 +803,9 b' mod tests {'
             .expect("expected valid config");
         let ret = config.get_byte_size(b"cmdserver", b"max-log-size");
         assert!(ret.is_ok(), "{:?}", ret);
+
+        let ret = config.get_byte_size(b"ui", b"formatted");
+        // FIXME should be `is_none()`
+        assert!(ret.unwrap().is_some());
     }
 }