Restore support for null in _AddServer_ method in ASP.NET integration (#1981)

* Upgrade ASP test project to dotnet8.

* Fix broken null support for _AddMqttServer_ method.

* Fix wrong code style

* Downgrade to dotnet7.0

* Expose certificate selection in MQTTnet options

* Revert "Expose certificate selection in MQTTnet options"

This reverts commit f7d098a119.

* Update release notes

Source/MQTTnet.AspnetCore/ServiceCollectionExtensions.cs

@@ -40,13 +40,10 @@ namespace MQTTnet.AspNetCore
                 throw new ArgumentNullException(nameof(services));
             }
 
-            if (configure == null)
-            {
-                throw new ArgumentNullException(nameof(configure));
-            }
-
             var serverOptionsBuilder = new MqttServerOptionsBuilder();
-            configure.Invoke(serverOptionsBuilder);
+
+            configure?.Invoke(serverOptionsBuilder);
+
             var options = serverOptionsBuilder.Build();
 
             return AddHostedMqttServer(services, options);