fix: high cpu issue (https://vimvq1987.com/fatal-flaw-with-geta-notfoundhandler/)

Author: valdis

src/Geta.NotFoundHandler/Core/Redirects/CustomRedirectCollection.cs

@@ -124,7 +124,7 @@ private CustomRedirect FindInternal(string url)
         foreach (var redirectPair in redirectsZA)
         {
             var oldUrl = redirectPair.Key;
-            if (oldUrl is null)
+            if (string.IsNullOrEmpty(oldUrl))
             {
                 continue;
             }