Description
The following sends a 0 value for cancel_time in the downtime history if the downtime wasn't actually cancelled:
https://github.com/Icinga/icinga2/blob/master/lib/icingadb/icingadb-objects.cpp#L1923
This will then write a 0 timestamp to the corresponding column in the database rather than a NULL value. Icinga 2 should either write null to Redis or omit the key in the JSON altogether if the downtime wasn't cancelled.