Fixed inconsistency between Delivery and Preview API for `locale=*` calls

We fixed an inconsistency between the preview and delivery APIs which led to unability to resolve linked entries when fetching entries using locale=*.

When calling the https://preview.contentful.com/spaces/{spaceid}/entries/?locale=* endpoint, the locale property won't be present in the sys, just like when calling https://cdn.contentful.com/spaces/{spaceid}/entries/?locale=*.