[ML] Unexpected interim results after advancing time into new empty bucket

[dimitris-athanasiou]

How to reproduce

1. Create a job with simple count detector and a bucket span of 5m
2. Run some data through the job up to the end of a bucket (using the end parameter of the start datafeed API)
3. Open the job again (it should have been auto-closed from step 2)
4. Call the flush API:

POST _xpack/anomaly_detectors/{job_id}/flush?advance_time={time}&calc_interim=true

where {time} should be a timestamp into the current bucket. E.g., if end was 2018-12-01T00:00:00Z, {time} should be 2018-12-01T00:00:01Z

Observed Behaviour

If you get the anomaly records, you should see a record which is interim and has an actual value of 0.0. This shouldn't have been created. Interestingly, calling step 4 with {time} being one millisecond forward makes that record disappear.

Also, this is broken since version 6.4.

[dimitris-athanasiou]

For future reference, it might be useful to note this bug was introduced by #97