Make startup config nullable when configuring app configuration

Jellyfin.Server/Program.cs

@@ -499,7 +499,7 @@ namespace Jellyfin.Server
                 .Build();
         }
 
-        private static IConfigurationBuilder ConfigureAppConfiguration(this IConfigurationBuilder config, IApplicationPaths appPaths, IConfiguration startupConfig = null)
+        private static IConfigurationBuilder ConfigureAppConfiguration(this IConfigurationBuilder config, IApplicationPaths appPaths, IConfiguration? startupConfig = null)
         {
             // Use the swagger API page as the default redirect path if not hosting the jellyfin-web content
             var inMemoryDefaultConfig = ConfigurationOptions.DefaultConfiguration;