GH-686: Handle tombstones in Json Ser/Deser

Fixes #686

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

(cherry picked from commit 8dc1dcd)

# Conflicts:
#	spring-kafka/src/main/java/org/springframework/kafka/support/serializer/JsonDeserializer.java
#	spring-kafka/src/main/java/org/springframework/kafka/support/serializer/JsonSerializer.java

(cherry picked from commit 02d4a45)

Author: garyrussell

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

@@ -77,6 +77,9 @@ public void configure(Map<String, ?> configs, boolean isKey) {
 	}
 
 	public T deserialize(String topic, byte[] data) {
+		if (data == null) {
+			return null;
+		}
 		if (this.reader == null) {
 			this.reader = this.objectMapper.readerFor(this.targetType);
 		}

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

@@ -56,6 +56,9 @@ public void configure(Map<String, ?> configs, boolean isKey) {
 	}
 
 	public byte[] serialize(String topic, T data) {
+		if (data == null) {
+			return null;
+		}
 		try {
 			byte[] result = null;
 			if (data != null) {