Fix event_mon exception on expired task.
Fix event_mon exception on expired task.

Setting None to event.proto.swarming_task_event.exit_code throws TypeError.
This explains why no CANCELED or EXPIRED task were surfaced.

Found via Stackdriver report.

BUG=

Review-Url: https://codereview.chromium.org/2891083002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py