Now use empty string for invariant in published

src/Umbraco.Web/PublishedCache/XmlPublishedCache/XmlPublishedContent.cs

@@ -150,7 +150,7 @@ namespace Umbraco.Web.PublishedCache.XmlPublishedCache
             }
         }
 
-        public override PublishedCultureInfos GetCulture(string culture = ".") => throw new NotSupportedException();
+        public override PublishedCultureInfos GetCulture(string culture = null) => throw new NotSupportedException();
 
         public override IReadOnlyDictionary<string, PublishedCultureInfos> Cultures => throw new NotSupportedException();