Monitoring's ClusterStatsCollector should be able to deal with a null license.

[martijnvg]

NPE caught in test failures (#52197):

ERROR][o.e.x.m.c.c.ClusterStatsCollector] [integTest-0] collector [cluster_stats] failed to collect data
»  java.lang.NullPointerException: null
»       at org.elasticsearch.xpack.monitoring.collector.cluster.ClusterStatsCollector.doCollect(ClusterStatsCollector.java:108) ~[x-pack-monitoring-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
»       at org.elasticsearch.xpack.monitoring.collector.Collector.collect(Collector.java:88) [x-pack-monitoring-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
»       at org.elasticsearch.xpack.monitoring.MonitoringService$MonitoringExecution$1.doRun(MonitoringService.java:251) [x-pack-monitoring-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
»       at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
»       at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) [?:?]
»       at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
»       at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:633) [elasticsearch-7.7.0-SNAPSHOT.jar:7.7.0-SNAPSHOT]
»       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
»       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
»       at java.lang.Thread.run(Thread.java:830) [?:?]

The ClusterStatsCollector#doCollect(...) (line 108) method should be able to handle a null License instance.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Monitoring)