Task property EndTime seems to have Nullable="false". This seems strange for an ongoing Task which should not have an EndTime yet. EndTime property of Job has Nullable="true", which I assume should be similar application of EndTime. Is this a fault in the Task CSDL? I would propose to change to Nullable="true" also for EndTime in Task in such case.
(I think also that the word "last" is a bit confusing in the Description of StartTime and EndTime. How could a Task be restarted? I would assume that each execution of an asynchronous operation should result in creation of new Task resource. OK?)
In the case where a Task is currently ongoing, I would expect "EndTime" to simply not be returned in the response payload. This is much like the case where you might have a DIMM not populated in a system, so properties like PartNumber, SerialNumber, and CapacityMiB are all not returned since the "State" of the resource is "Absent". Using null for the property value traditionally means that the property should be filled in by the service, but the service is unable to ascertain the value for the property.