Apply suggestions from code review

Co-Authored-By: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Web/Security/AppBuilderExtensions.cs

@@ -430,7 +430,7 @@ namespace Umbraco.Web.Security
         {
             if (app == null)
             {
-                throw new ArgumentNullException("app");
+                throw new ArgumentNullException(nameof(app));
             }
             return app.CreatePerOwinContext(createCallback, (options, instance) => instance.Dispose());
         }

src/Umbraco.Web/Security/IdentityFactoryMiddleware.cs

@@ -22,7 +22,7 @@ namespace Umbraco.Web.Security
         {
             if (options == null)
             {
-                throw new ArgumentNullException("options");
+                throw new ArgumentNullException(nameof(options));
             }
             if (options.Provider == null)
             {

src/Umbraco.Web/Security/UmbracoSecurityStampValidator.cs

@@ -24,11 +24,7 @@ namespace Umbraco.Web.Security
 
             return async context =>
             {
-                var currentUtc = DateTimeOffset.UtcNow;
-                if (context.Options != null && context.Options.SystemClock != null)
-                {
-                    currentUtc = context.Options.SystemClock.UtcNow;
-                }
+                 var currentUtc = context.Options?.SystemClock?.UtcNow ?? DateTimeOffset.UtcNow;
 
                 var issuedUtc = context.Properties.IssuedUtc;