Add supportsMimeType to KafkaNullConverter

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/config/BinderFactoryAutoConfiguration.java

@@ -256,6 +256,11 @@ public class BinderFactoryAutoConfiguration {
 			return "org.springframework.kafka.support.KafkaNull".equals(aClass.getName());
 		}
 
+		@Override
+		protected boolean supportsMimeType(@Nullable MessageHeaders headers) {
+			return true;
+		}
+
 		@Override
 		protected boolean canConvertFrom(Message<?> message, Class<?> targetClass) {
 			return message.getPayload().getClass().getName().equals("org.springframework.kafka.support.KafkaNull");