Fix varargs warn for CompositeProducerListener

**Cherry-pick to 2.1.x**

Author: artembilan

Diff for: spring-kafka/src/main/java/org/springframework/kafka/support/CompositeProducerListener.java

@@ -30,7 +30,9 @@
  *
  * @param <K> the key type.
  * @param <V> the value type.
+ *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 2.1.6
  *
@@ -40,13 +42,16 @@ public class CompositeProducerListener<K, V> implements ProducerListener<K, V> {
 	private final List<ProducerListener<K, V>> delegates = new CopyOnWriteArrayList<>();
 
 	@SafeVarargs
+	@SuppressWarnings("varargs")
 	public CompositeProducerListener(ProducerListener<K, V>... delegates) {
 		setDelegates(delegates);
 	}
 
 	@SafeVarargs
+	@SuppressWarnings("varargs")
 	public final void setDelegates(ProducerListener<K, V>... delegates) {
 		Assert.notNull(delegates, "'delegates' cannot be null");
+		Assert.noNullElements(delegates, "'delegates' cannot contain null elements");
 		this.delegates.clear();
 		this.delegates.addAll(Arrays.asList(delegates));
 	}

Diff for: spring-kafka/src/test/java/org/springframework/kafka/core/KafkaTemplateTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2017 the original author or authors.
+ * Copyright 2016-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -268,6 +268,7 @@ public void onError(ProducerRecord<Integer, String> producerRecord, Exception ex
 		}
 		PL pl1 = new PL();
 		PL pl2 = new PL();
+		@SuppressWarnings("unchecked")
 		CompositeProducerListener<Integer, String> cpl = new CompositeProducerListener<>(pl1, pl2);
 		template.setProducerListener(cpl);
 		template.sendDefault("foo");