[#1671] Fix bug in ChunkedWriteHandler which produce a NPE on empty chunks

handler/src/main/java/io/netty/handler/stream/ChunkedWriteHandler.java

@@ -15,6 +15,7 @@
  */
 package io.netty.handler.stream;
 
+import io.netty.buffer.Unpooled;
 import io.netty.channel.Channel;
 import io.netty.channel.ChannelDuplexHandler;
 import io.netty.channel.ChannelFuture;
@@ -260,6 +261,12 @@ public class ChunkedWriteHandler
                     break;
                 }
 
+                if (message == null) {
+                    // If message is null write an empty ByteBuf.
+                    // See https://github.com/netty/netty/issues/1671
+                    message = Unpooled.EMPTY_BUFFER;
+                }
+
                 pendingWrites.incrementAndGet();
                 ChannelFuture f = ctx.write(message);
                 if (endOfInput) {