Fixed deadlock in class initialization.

jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/EmptyHttpFields.java

@@ -0,0 +1,26 @@
+//
+// ========================================================================
+// Copyright (c) 1995 Mort Bay Consulting Pty Ltd and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+// which is available at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+// ========================================================================
+//
+
+package org.eclipse.jetty.http;
+
+import java.util.Collections;
+import java.util.ListIterator;
+
+class EmptyHttpFields implements HttpFields
+{
+    @Override
+    public ListIterator<HttpField> listIterator(int index)
+    {
+        return Collections.emptyListIterator();
+    }
+}

jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java

@@ -64,7 +64,7 @@ public interface HttpFields extends Iterable<HttpField>, Supplier<HttpFields>
     /**
      * <p>A constant for an immutable and empty {@link HttpFields}.</p>
      */
-    HttpFields EMPTY = build().asImmutable();
+    HttpFields EMPTY = new EmptyHttpFields();
 
     /**
      * <p>Returns an empty {@link Mutable} instance.</p>