[fix][broker] Fix to avoid TopicStatsImpl NPE even if producerName is null

[equanz]

Fixes #15471

Motivation

Please see #15471 .

Modifications

• if producerName is null, then fall back to supportsPartialProducer=false
  • currently, producerName is required whensupportsPartialProducer=true, because stats are aggregated by producerName
• allow null producerName on getPublishers

Verifying this change

• Make sure that the change passes the CI checks.

• Add tests for null producerName

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

Check the box below or label this PR directly.

Need to update docs?

• no-need-doc

This PR is one of the fixing issues.

[Technoboy-]

Why the produceName is null?

[Technoboy-]

Why the produceName is null?

Get it from the issue comment.

[Technoboy-]

I think the root cause is here :

pulsar/pulsar-common/src/main/java/org/apache/pulsar/common/policies/data/stats/TopicStatsImpl.java

Lines 234 to 238 in ca614e0

	if (this.publishers.size() != stats.publishers.size()) {
	for (int i = 0; i < stats.publishers.size(); i++) {
	PublisherStatsImpl newStats = new PublisherStatsImpl();
	newStats.setSupportsPartialProducer(false);
	this.publishers.add(newStats.add(stats.publishers.get(i)));

We should set producerName for the newStats

[equanz]

I think the root cause is here :

Yes. I think so too.

We should set producerName for the newStats

I think we can't set producerName in this section, because old client's partitioned producer can have different producerName between internal producers. #12401

[Technoboy-]

I think the root cause is here :

Yes. I think so too.

We should set producerName for the newStats

I think we can't set producerName in this section, because old client's partitioned producer can have different producerName between internal producers. #12401

Ok, make sense.