=str #15755 #15756 source and sink

sorry, deleted the old branch by accident, previous discussion was at #15786 
I don't completely understand this. Where is an identityProcessor injected? You mean if ops.isEmpty is true? 
I ask because the only reason why that identityProcessor is needed for empty ops if you materialize an empty Duct, but if at least one side is bound that identityProcessor is not needed.