Allow unit struct to be flattened as empty object

Author: gustavo-shigueo

macros/src/types/unit.rs

@@ -13,7 +13,7 @@ pub(crate) fn empty_object(attr: &StructAttr, ts_name: Expr) -> DerivedTS {
     DerivedTS {
         crate_rename: crate_rename.clone(),
         inline: quote!("Record<symbol, never>".to_owned()),
-        inline_flattened: None,
+        inline_flattened: Some(quote!("{ }".to_owned())),
         docs: attr.docs.clone(),
         dependencies: Dependencies::new(crate_rename),
         export: attr.export,