Is it possible to fetch Unassigned value for PlannedFor field in RTC using OSLC query or reportable API or IBM sdk?

I am trying to fetch Unassigned value for PlannedFor field in RTC using OSLC query or reportable API or IBM sdk,actually the scenario here is that i am trying to update story using input xml and payload.Input xml has PlannedFor field as blank ,now when i use http PUT method to update the plannedFor field in RTC its not getting updated to Unassigned value ,So please suggest me if there is any possible way to get the value using OSLC ,reportable API or IBM sdk.Awaiting for the response.