GH-615: Fix new Sonar Issue

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/listener/DefaultErrorHandler.java

@@ -89,7 +89,7 @@ public DefaultErrorHandler(@Nullable ConsumerRecordRecoverer recoverer, BackOff
 		super(recoverer, backOff, createFallback(backOff, recoverer));
 	}
 
-	private static CommonErrorHandler createFallback(BackOff backOff, ConsumerRecordRecoverer recoverer) {
+	private static CommonErrorHandler createFallback(BackOff backOff, @Nullable ConsumerRecordRecoverer recoverer) {
 		return new ErrorHandlerAdapter(new RetryingBatchErrorHandler(backOff, recoverer));
 	}