Fixed "missing" listeners on addListeners method in Netty instrumenta…

…tion. Add null check validation for Netty listener advice apply

instrumentation/netty/netty-4-common/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v4/common/FutureListenerWrappers.java

@@ -43,7 +43,7 @@ protected Boolean computeValue(Class<?> type) {
       };
 
   public static boolean shouldWrap(GenericFutureListener<? extends Future<?>> listener) {
-    return shouldWrap.get(listener.getClass());
+    return listener != null && shouldWrap.get(listener.getClass());
   }
 
   @SuppressWarnings("unchecked")

instrumentation/netty/netty-4-common/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/netty/v4/common/NettyFutureInstrumentation.java

@@ -82,6 +82,8 @@ public static void wrapListener(
       for (int i = 0; i < listeners.length; ++i) {
         if (FutureListenerWrappers.shouldWrap(listeners[i])) {
           wrappedListeners[i] = FutureListenerWrappers.wrap(context, listeners[i]);
+        } else {
+          wrappedListeners[i] = listeners[i];
         }
       }
       listeners = wrappedListeners;