Support null executor in NettyResponseFuture#addListener, close #1369 (#1371)

Author: joedj

client/src/main/java/org/asynchttpclient/netty/NettyResponseFuture.java

@@ -256,6 +256,9 @@ public void touch() {
 
     @Override
     public ListenableFuture<V> addListener(Runnable listener, Executor exec) {
+        if (exec == null) {
+            exec = Runnable::run;
+        }
         future.whenCompleteAsync((r, v) -> listener.run(), exec);
         return this;
     }