add comment about cassandra nil vs empty struct

common/persistence/nosql/nosqlplugin/cassandra/tasks.go

@@ -90,6 +90,7 @@ func toTaskListPartitionConfig(v interface{}) *persistence.TaskListPartitionConf
 	numRead := partition["num_read_partitions"].(int)
 	numWrite := partition["num_write_partitions"].(int)
 
+	// even if null is stored in cassandra, gocql will return empty struct instead of nil
 	if version == 0 && numRead == 0 && numWrite == 0 {
 		return nil
 	}