MINOR: Fix leader param description in TopicPartitionInfo constructor

[mingyen066]

The description is inconsistent with method leader(). After checking
the code, we always use pass null to TopicPartitionInfo instead of
noNode The current description is inconsistent with the leader()
method. After reviewing the code, we actually pass null to
TopicPartitionInfo instead of using noNode() in KafkaAdminClient

        for (PartitionInfo partitionInfo : partitionInfos) {
            TopicPartitionInfo topicPartitionInfo = new
TopicPartitionInfo(
                partitionInfo.partition(), leader(partitionInfo),
Arrays.asList(partitionInfo.replicas()),
                Arrays.asList(partitionInfo.inSyncReplicas()));
            partitions.add(topicPartitionInfo);
        }

    private Node leader(PartitionInfo partitionInfo) {
        if (partitionInfo.leader() == null ||
partitionInfo.leader().id() == Node.noNode().id())
            return null;
        return partitionInfo.leader();
    }

https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/admin/KafkaAdminClient.java#L2440-L2454

[squah-confluent]

There are a few places we pass Node.noNode() in in tools/src/test. We're not very consistent in usage of null vs Node.noNode(). The new param description doesn't look fully accurate either?