[broker] Fix NPE that occurs in PersistentStickyKeyDispatcherMultipleConsumers when debug log is enabled

[massakam]

Motivation

#8969 fixed an issue where NPE could occur in PersistentStickyKeyDispatcherMultipleConsumers. However, if the log level is debug, I think NPE will occur in the following part as well:

pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentStickyKeyDispatcherMultipleConsumers.java

Lines 183 to 184 in 2fd878a

	log.debug("[{}] select consumer {} with messages num {}, read type is {}",
	name, consumer.consumerName(), messagesForC, readType);

Modifications

If consumer is null, "null" will be output to the log as the consumer name.

[eolivelli]

LGTM