Added schema by default and reference it from the appsettings files.

+ Do not suggest null values instead of objects

src/Umbraco.Web.UI.NetCore/appsettings.json

@@ -1,4 +1,5 @@
 {
+  "$schema" : "./umbraco/config/appsettings-schema.json",
   "ConnectionStrings": {
     "umbracoDbDSN": ""
   },
@@ -71,4 +72,4 @@
       }
     }
   }
-}
+}