Update to work with nil check over len

This is to stay consistent with other areas of the codebase
(e.g. toESModuleExports). There could be a valid case where we want
named to be an empty slice and default be non-nil, and not use this
feature to convert from default to namespace exports.

js/modules/gomodule.go

@@ -26,7 +26,7 @@ func (gm *goModule) Instantiate(rt *sobek.Runtime) (sobek.CyclicModuleInstance,
 		named := mi.Exports().Named
 
 		switch {
-		case len(named) == 0 && mi.Exports().Default != nil:
+		case named == nil && mi.Exports().Default != nil:
 			// If named length is 0 but default is defined, then try to work with
 			// default and extract the names of the object's properties. This
 			// behavior isn't ESM compatible, but we do want to allow defaults to