Handling optional ErrorCluster argument in APIs is not consistent

[sanmut]

QueueRef_Obtain checks whether the error pointer is null (to see whether the error cluster is being indeed passed and not marked as a wildcard). However, UnflattenFromJSON does not do the same check but distinguishes whether error cluster is being passed or not by checking the number of arguments. This is error prone and makes codegen difficult to handle situations differently. When error cluster is passed as wild card to UnflattenFromJSON, this is in fact not null -- which is confusing. We should fix this to follow the pattern of QueueRef_Obtain.