For scenarios where the incoming traffic of all input partitions are small, there's a pitfall that the enforced processing timer is not reset after we have enforce processed ALL records. The fix itself is pretty simple: we just reset the timer when there's no buffered records.
Reviewers: Javier Holguera <javier.holguera@gmail.com>, Boyang Chen <boyang@confluent.io>, Bill Bejeck <bill@confluent.io>