Fix incompatible with caller stubbing

https://build.spring.io/browse/SK-SK20X-43/

Author: artembilan

spring-kafka/src/test/java/org/springframework/kafka/listener/TransactionalContainerTests.java

@@ -186,7 +186,7 @@ public void testConsumeAndProduceTransactionRollback() throws Exception {
 			return null;
 		}).given(consumer).seek(any(), anyLong());
 		ConsumerFactory cf = mock(ConsumerFactory.class);
-		willReturn(consumer).given(cf).createConsumer("group", "");
+		willReturn(consumer).given(cf).createConsumer("group", null);
 		Producer producer = mock(Producer.class);
 		final CountDownLatch closeLatch = new CountDownLatch(1);
 		willAnswer(i -> {