8263899: HttpClient throws NPE in AuthenticationFilter when parsing www-authenticate head

test/jdk/java/net/httpclient/EmptyAuthenticate.java

@@ -41,13 +41,15 @@
 public class EmptyAuthenticate {
 
     public static void main(String[] args) throws IOException, URISyntaxException, InterruptedException {
-        int port = 8000;
+        int port = 0;
 
         //start server:
         HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);
+        port = server.getAddress().getPort();
         server.createContext("/", exchange -> {
             String response = "test body";
-            exchange.getResponseHeaders().add("www-authenticate", ""); //this empty header will make the HttpClient crash
+            //this empty header will make the HttpClient throw NPE
+            exchange.getResponseHeaders().add("www-authenticate", ""); 
             exchange.sendResponseHeaders(401, response.length());
             OutputStream os = exchange.getResponseBody();
             os.write(response.getBytes());