Fix code smell in websocket and webflux modules (#2669)

* * Fix code smell in websocket and webflux modules

* * More fixes

* * Fix NPE in the `IntegrationHandlerResultHandler`

spring-integration-core/src/main/java/org/springframework/integration/transformer/PayloadTypeConvertingTransformer.java

@@ -53,7 +53,7 @@ public class PayloadTypeConvertingTransformer<T, U> extends AbstractPayloadTrans
 	}
 
 	@Override
-	protected void onInit() throws Exception {
+	protected void onInit() throws Exception { // NOSONAR thrown by super class
 		super.onInit();
 		Assert.notNull(this.converter, () -> getClass().getName() + " requires a Converter<Object, Object>");
 	}