Prevent NullPointerException in TransportRolloverAction

[dakrone]

It's possible for the passed in IndexMetaData to be null (for
instance, cluster state passed in does not have the index in its
metadata) which in turn can cause a NullPointerException when
evaluating the conditions for an index. This commit adds null protection
and unit tests for this case.

Resolves #43296

[elasticmachine]

Pinging @elastic/es-core-features

[martijnvg]

LGTM

I don't think we need to backport top the 7.0 branch? That version line is no longer maintained.
Does this problem also exist in 6.8.0? If so then I think we should also backport to the 6.x branch.

[dnhatn]

Thanks @dakrone. I left a question.

[dnhatn]

LGTM

[cdenneen]

@dakrone still seeing this in 6.8.6 is there any chance this was possibly reintroduced back after the 6.8.3 fix?

{
  "indices" : {
    "shrink-logstash-apfeed-000361" : {
      "index" : "shrink-logstash-apfeed-000361",
      "managed" : true,
      "policy" : "logging_policy",
      "lifecycle_date_millis" : 1599708150241,
      "phase" : "warm",
      "action" : "forcemerge",
      "action_time_millis" : 1599722444069,
      "step" : "ERROR",
      "step_time_millis" : 1600195055113,
      "failed_step" : "segment-count",
      "step_info" : {
        "type" : "null_pointer_exception",
        "reason" : null,
        "stack_trace" : """
java.lang.NullPointerException
	at org.elasticsearch.xpack.core.indexlifecycle.SegmentCountStep.lambda$evaluateCondition$2(SegmentCountStep.java:45)
	at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:61)
	at org.elasticsearch.action.support.ContextPreservingActionListener.onResponse(ContextPreservingActionListener.java:43)
	at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:85)
	at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:81)
	at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction.onCompletion(TransportBroadcastByNodeAction.java:393)
	at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction.onNodeFailure(TransportBroadcastByNodeAction.java:378)
	at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction$1.handleException(TransportBroadcastByNodeAction.java:339)
	at org.elasticsearch.transport.TransportService$ContextRestoreResponseHandler.handleException(TransportService.java:1114)
	at org.elasticsearch.transport.TransportService.lambda$onConnectionClosed$8(TransportService.java:963)
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:681)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

"""
      }
    }
  }
}