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
if (ordinal == null) {
            keyToSequences.clear();
        } else {
            
            keyToSequences.trimToTail(ordinal);
        }
    }
    public void addToCompleted(Sequence sequence) {
        this.completed.add(sequence);
    }
    Set<Sequence> toCheckForMissing() {
        return toCheckForMissing;
    }
    public void clear() {
        stats.clear();
        keyToSequences.clear();
        stageToKeys.clear();
        completed.clear();
        toCheckForMissing.clear();
        clearCircuitBreaker();
    }
    
    protected long ramBytesUsedInFlight() {
        return RamUsageEstimator.sizeOf(keyToSequences) + RamUsageEstimator.sizeOf(stageToKeys);
    }
    
    protected long ramBytesUsedCompleted() {
        return RamUsageEstimator.sizeOfCollection(completed);
    }