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/Sequence.java
return key;
    }
    public Ordinal ordinal() {
        return matches[currentStage].ordinal();
    }
    public Ordinal startOrdinal() {
        return matches[0].ordinal();
    }
    public List<HitReference> hits() {
        List<HitReference> hits = new ArrayList<>(matches.length);
        for (Match m : matches) {
            hits.add(m == null ? null : m.hit());
        }
        return hits;
    }
    @Override
    public long ramBytesUsed() {
        return SHALLOW_SIZE + RamUsageEstimator.sizeOf(key) + RamUsageEstimator.sizeOf(matches);
    }
    @Override
    public int compareTo(Sequence o) {
        int result = ordinal().compareTo(o.ordinal());
        if (result == 0) {
            return key().compareTo(o.key());
        }
        return result;
    }
    @Override
    public int hashCode() {
        return Objects.hash(currentStage, key);