Issue #3597 NPE in DumpableCollection

Signed-off-by: Jan Bartel <janb@webtide.com>

jetty-util/src/main/java/org/eclipse/jetty/util/component/DumpableCollection.java

@@ -53,7 +53,7 @@ public class DumpableCollection implements Dumpable
     @Override
     public void dump(Appendable out, String indent) throws IOException
     {
-        Object[] array = _collection.toArray();
-        Dumpable.dumpObjects(out,indent,_name + " size="+array.length, array);
+        Object[] array = (_collection == null ? null : _collection.toArray());
+        Dumpable.dumpObjects(out,indent,_name + " size="+(array == null ? 0 : array.length), array);
     }
 }

jetty-util/src/test/java/org/eclipse/jetty/util/component/DumpableCollectionTest.java

@@ -0,0 +1,54 @@
+//
+//  ========================================================================
+//  Copyright (c) 1995-2019 Mort Bay Consulting Pty. Ltd.
+//  ------------------------------------------------------------------------
+//  All rights reserved. This program and the accompanying materials
+//  are made available under the terms of the Eclipse Public License v1.0
+//  and Apache License v2.0 which accompanies this distribution.
+//
+//      The Eclipse Public License is available at
+//      http://www.eclipse.org/legal/epl-v10.html
+//
+//      The Apache License v2.0 is available at
+//      http://www.opensource.org/licenses/apache2.0.php
+//
+//  You may elect to redistribute this code under either of these licenses.
+//  ========================================================================
+//
+
+package org.eclipse.jetty.util.component;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import java.util.ArrayList;
+import java.util.Collection;
+
+import org.hamcrest.Matchers;
+import org.junit.jupiter.api.Test;
+
+public class DumpableCollectionTest
+{
+
+    @Test
+    public void testNullDumpableCollection () throws Exception
+    {
+        DumpableCollection dc = new DumpableCollection("null test", null);
+        String dump = dc.dump();
+        assertThat(dump, Matchers.containsString("size=0"));
+    }
+    
+    @Test
+    public void testNonNullDumpableCollection () throws Exception
+    {
+        Collection<String> collection = new ArrayList<>();
+        collection.add("one");
+        collection.add("two");
+        collection.add("three");
+        
+        DumpableCollection dc = new DumpableCollection("non null test", collection);
+        String dump = dc.dump();
+        assertThat(dump, Matchers.containsString("one"));
+        assertThat(dump, Matchers.containsString("two"));
+        assertThat(dump, Matchers.containsString("three"));
+    }
+}