Add @nullable for ProducerFactory & ConsumerFactory interfaces to enable null records. (#4367)

Fixes: #4355

**Auto-cherry-pick to `4.0.x`**

Signed-off-by: jad837 <jadhavsaurabh037@gmail.com>

Author: jad837

spring-kafka/src/main/java/org/springframework/kafka/core/ConsumerFactory.java

@@ -34,7 +34,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  */
-public interface ConsumerFactory<K, V> {
+public interface ConsumerFactory<K, V extends @Nullable Object> {
 
 	/**
 	 * Create a consumer with the group id and client id as configured in the properties.

spring-kafka/src/main/java/org/springframework/kafka/core/ProducerFactory.java

@@ -36,7 +36,7 @@
  * @author Thomas Strauß
  * @author  Kwon YongHyun
  */
-public interface ProducerFactory<K, V> {
+public interface ProducerFactory<K, V extends @Nullable Object> {
 
 	/**
 	 * The default close timeout duration as 30 seconds.