GH-686: Handle tombstones in Json Ser/Deser

Fixes #686

**Cherry-pick to 2.1.x, 2.0.x, 1.3.x**

Author: garyrussell

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

@@ -215,6 +215,9 @@ private void addTargetPackageToTrusted() {
 
 	@Override
 	public T deserialize(String topic, Headers headers, byte[] data) {
+		if (data == null) {
+			return null;
+		}
 		JavaType javaType = this.typeMapper.toJavaType(headers);
 		if (javaType == null) {
 			Assert.state(this.targetType != null, "No type information in headers and no default type provided");
@@ -233,6 +236,9 @@ public T deserialize(String topic, Headers headers, byte[] data) {
 
 	@Override
 	public T deserialize(String topic, byte[] data) {
+		if (data == null) {
+			return null;
+		}
 		if (this.reader == null) {
 			this.reader = this.objectMapper.readerFor(this.targetType);
 		}

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

@@ -130,6 +130,9 @@ else if (config instanceof String) {
 
 	@Override
 	public byte[] serialize(String topic, Headers headers, T data) {
+		if (data == null) {
+			return null;
+		}
 		if (this.addTypeInfo && headers != null) {
 			this.typeMapper.fromJavaType(this.objectMapper.constructType(data.getClass()), headers);
 		}
@@ -138,6 +141,9 @@ public byte[] serialize(String topic, Headers headers, T data) {
 
 	@Override
 	public byte[] serialize(String topic, T data) {
+		if (data == null) {
+			return null;
+		}
 		try {
 			byte[] result = null;
 			if (data != null) {