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
stageToKeys.add(stage, key);
            keyToSequences.add(stage, sequence);
        }
    }
    public void tryComplete(Sequence sequence) {
        if (missingEventStagesExist) {
            toCheckForMissing.add(sequence);
        } else {
            completed.add(sequence);
        }
    }
    private void calculateHeadLimit() {
        headLimit = limit != null && limit.limit() > 0 && completed.size() == limit.totalLimit();
    }
    int previousPositiveStage(int stage) {
        for (int i = stage - 1; i >= 0; i--) {
            if (missingEventStages[i] == false) {
                return i;
            }
        }
        return -1;
    }
    public boolean limitReached() {
        calculateHeadLimit();
        return headLimit;
    }
    int nextPositiveStage(int stage) {
        for (int i = stage + 1; i < missingEventStages.length; i++) {
            if (missingEventStages[i] == false) {
                return i;