Allow null method for getTransactionAttribute

Update MatchAlwaysTransactionAttributeSource.getTransactionAttribute
to allow a null method argument. Passing a null method is not
recommended and is not indicated as valid in the Javadoc, however,
this was allowed in previous versions of Spring.

Issue: SPR-11048

spring-tx/src/main/java/org/springframework/transaction/interceptor/MatchAlwaysTransactionAttributeSource.java

@@ -53,7 +53,7 @@ public class MatchAlwaysTransactionAttributeSource implements TransactionAttribu
 
 	@Override
 	public TransactionAttribute getTransactionAttribute(Method method, Class<?> targetClass) {
-		return (ClassUtils.isUserLevelMethod(method) ? this.transactionAttribute : null);
+		return (method == null || ClassUtils.isUserLevelMethod(method) ? this.transactionAttribute : null);
 	}