Handle the fact that ShardStats instance may have no commit or seqno stats

The should fix the following NPE:

```
[2019-02-11T23:27:48,452][WARN ][o.e.p.PersistentTasksNodeService] [node_s_0] task kD8YzUhHTK6uKNBNQI-1ZQ-0 failed with an exception
  1> java.lang.NullPointerException: null
  1>    at org.elasticsearch.xpack.ccr.action.ShardFollowTasksExecutor.lambda$fetchFollowerShardInfo$7(ShardFollowTasksExecutor.java:305) ~[main/:?]
  1>    at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:61) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:68) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.TransportAction$1.onResponse(TransportAction.java:64) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction.onCompletion(TransportBroadcastByNodeAction.java:383) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction.onNodeResponse(TransportBroadcastByNodeAction.java:352) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction$1.handleResponse(TransportBroadcastByNodeAction.java:324) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$AsyncAction$1.handleResponse(TransportBroadcastByNodeAction.java:314) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.TransportService$ContextRestoreResponseHandler.handleResponse(TransportService.java:1108) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.TransportService$DirectResponseChannel.processResponse(TransportService.java:1189) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.TransportService$DirectResponseChannel.sendResponse(TransportService.java:1169) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.TaskTransportChannel.sendResponse(TaskTransportChannel.java:54) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$BroadcastByNodeTransportRequestHandler.messageReceived(TransportBroadcastByNodeAction.java:417) [elasticsearch-8.0.0-SNAP
SHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.action.support.broadcast.node.TransportBroadcastByNodeAction$BroadcastByNodeTransportRequestHandler.messageReceived(TransportBroadcastByNodeAction.java:391) [elasticsearch-8.0.0-SNAP
SHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:63) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.transport.TransportService$7.doRun(TransportService.java:687) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:751) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
  1>    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_202]
  1>    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_202]
  1>    at java.lang.Thread.run(Thread.java:748) [?:1.8.0_202]
```

Relates to elastic#38779

Author: martijnvg

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardFollowTasksExecutor.java

@@ -9,6 +9,7 @@
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.UnavailableShardsException;
 import org.elasticsearch.action.admin.cluster.state.ClusterStateRequest;
 import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
 import org.elasticsearch.action.admin.indices.close.CloseIndexRequest;
@@ -302,9 +303,19 @@ private void fetchFollowerShardInfo(
             if (filteredShardStats.isPresent()) {
                 final ShardStats shardStats = filteredShardStats.get();
                 final CommitStats commitStats = shardStats.getCommitStats();
-                final String historyUUID = commitStats.getUserData().get(Engine.HISTORY_UUID_KEY);
-
+                if (commitStats == null) {
+                    // UnavailableShardsException will be retried byShardFollowNodeTask.shouldRetry(...)
+                    errorHandler.accept(new UnavailableShardsException(shardId, "commit_stats are missing"));
+                    return;
+                }
                 final SeqNoStats seqNoStats = shardStats.getSeqNoStats();
+                if (seqNoStats == null) {
+                    // UnavailableShardsException will be retried byShardFollowNodeTask.shouldRetry(...)
+                    errorHandler.accept(new UnavailableShardsException(shardId, "seq_no_stats are missing"));
+                    return;
+                }
+
+                final String historyUUID = commitStats.getUserData().get(Engine.HISTORY_UUID_KEY);
                 final long globalCheckpoint = seqNoStats.getGlobalCheckpoint();
                 final long maxSeqNo = seqNoStats.getMaxSeqNo();
                 handler.accept(historyUUID, globalCheckpoint, maxSeqNo);