Update marionette.layout.js

Layout.render assumes that if the view is closed, it has been previously rendered. This assumption is valid in a 100% Marionette application; however, when mid-migration to Marionette pre-existing view handling may not respect this invariant.

Without this change the regions are uninitalized in `onRender` despite the layout having been rendered.

I haven't prepared tests for this change yet, but this change resolved the associated bug in the application I am migrating to Marionette. I will add them when I get a chance.