Fix DefaultChannelPipeline.first() so that it returns null when only the sentinel handlers remain (this is how DefaultChannelPipeline.last() works already)

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -751,7 +751,7 @@ final class DefaultChannelPipeline implements ChannelPipeline {
     @Override
     public ChannelHandler first() {
         DefaultChannelHandlerContext first = head.next;
-        if (first == null) {
+        if (first == head) {
             return null;
         }
         return first.handler();