fix samesite bug from aspnet code to check for null userAgent

Author: jennyf19

Microsoft.Identity.Web/WebAppServiceCollectionExtensions.cs

@@ -206,6 +206,11 @@ private static void CheckSameSite(HttpContext httpContext, CookieOptions options
         // Method taken from https://devblogs.microsoft.com/aspnet/upcoming-samesite-cookie-changes-in-asp-net-and-asp-net-core/
         public static bool DisallowsSameSiteNone(string userAgent)
         {
+            if (string.IsNullOrEmpty(userAgent))
+            {
+                return false;
+            }
+
             // Cover all iOS based browsers here. This includes:
             // - Safari on iOS 12 for iPhone, iPod Touch, iPad
             // - WkWebview on iOS 12 for iPhone, iPod Touch, iPad