Don't overwrite TypeName if it has been set to null

In order to be able to use templates and at the same time remove
the doc type we shouldn't override it if it's set to null.

src/Serilog.Sinks.Elasticsearch/Sinks/ElasticSearch/ElasticsearchSinkState.cs

@@ -69,8 +69,8 @@ private ElasticsearchSinkState(ElasticsearchSinkOptions options)
             if (string.IsNullOrWhiteSpace(options.IndexFormat)) throw new ArgumentException("options.IndexFormat");
             if (string.IsNullOrWhiteSpace(options.TemplateName)) throw new ArgumentException("options.TemplateName");
 
-            // Strip type argument if ESv7 since multiple types are not supported anymore
-            if (options.AutoRegisterTemplateVersion == AutoRegisterTemplateVersion.ESv7)
+            // Since TypeName is deprecated we shouldn't set it, if has been deliberately set to null.
+            if (options.TypeName == null && options.AutoRegisterTemplateVersion == AutoRegisterTemplateVersion.ESv7)
             {
                 options.TypeName = "_doc";
             }