A query like 

```
sequence by user.name with maxspan=90m
![ authentication where ... ]
[ authentication where ...]
![ authentication where ... ]
[ authentication where ... ]
![ authentication where ... ]
```

can result in a NullPointerException

```
java.lang.NullPointerException: Cannot invoke "org.elasticsearch.xpack.eql.execution.sequence.Match.ordinal()" because the return value of "org.elasticsearch.xpack.eql.execution.sequence.Sequence.matchAt(int)" is null
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.prepareQueryForMissingEvents(TumblingWindow.java:313)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.checkMissingEvents(TumblingWindow.java:219)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.payload(TumblingWindow.java:717)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.completeBaseCriterion(TumblingWindow.java:499)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.baseCriterion(TumblingWindow.java:405)
        at org.elasticsearch.xpack.eql.execution.sequence.TumblingWindow.lambda$advance$6(TumblingWindow.java:356)
```
x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/execution/sequence/SequenceMatcher.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.eql.execution.sequence;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.lucene.util.RamUsageEstimator;
import org.elasticsearch.common.breaker.CircuitBreaker;
import org.elasticsearch.common.logging.LoggerMessageFormat;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.xpack.eql.execution.search.HitReference;
import org.elasticsearch.xpack.eql.execution.search.Limit;
import org.elasticsearch.xpack.eql.execution.search.Ordinal;
import org.elasticsearch.xpack.eql.execution.search.Timestamp;
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;
/**
 * Matcher of sequences. Keeps track of on-going sequences and advancing them through each stage.
 */
public class SequenceMatcher {
    private static final String CB_INFLIGHT_LABEL = "sequence_inflight";
    private static final String CB_COMPLETED_LABEL = "sequence_completed";
    private static final Logger log = LogManager.getLogger(SequenceMatcher.class);
    static class Stats {