ISPN-4460 Map-Reduce: Mapper sometimes receives null value

Master only. See https://issues.jboss.org/browse/ISPN-4460 for more details
Technically I wouldn't think this is possible since you always pass true for the value to the processOnAllStores method of the PassivationManager.  Have you seen a CacheLoader pass in a null value here in that case?  If so that seems like a bug elsewhere not in this code to me.