rust-config: fix incorrect coercion of null values to false

As explained in the previous changeset:

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.

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

@@ -198,7 +198,7 b' impl TryFrom<&DefaultConfigItem> for Opt'
                     _ => Err(err),
                 }
             }
-            None => Ok(Some(false)),
+            None => Ok(None),
         }
     }
 }

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

@@ -805,7 +805,6 b' mod tests {'
         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());
+        assert!(ret.unwrap().is_none());
     }
 }