Update sub to not break when str is undefined
Update sub to not break when str is undefined

For development reasons, I had a model without destroy defined, but I was calling `destroy()` on my test, canjs was breaking when calling sub with `str = undefined`.
sporto/canjs,juristr/canjs,bitovi/canjs,rasjani/canjs,beno/canjs,sporto/canjs,Psykoral/canjs,azazel75/canjs,shiftplanning/canjs,janza/canjs,thomblake/canjs,thecountofzero/canjs,Psykoral/canjs,rjgotten/canjs,tracer99/canjs,gsmeets/canjs,gsmeets/canjs,UXsree/canjs,rasjani/canjs,cohuman/canjs,Psykoral/canjs,dispatchrabbi/canjs,sporto/canjs,rjgotten/canjs,cohuman/canjs,cohuman/canjs,juristr/canjs,bitovi/canjs,patrick-steele-idem/canjs,Psykoral/canjs,rjgotten/canjs,beno/canjs,yusufsafak/canjs,whitecolor/canjs,ackl/canjs,yusufsafak/canjs,tracer99/canjs,airhadoken/canjs,tracer99/canjs,patrick-steele-idem/canjs,janza/canjs,beno/canjs,asavoy/canjs,rasjani/canjs,scorphus/canjs,azazel75/canjs,rasjani/canjs,patrick-steele-idem/canjs,shiftplanning/canjs,bitovi/canjs,thomblake/canjs,gsmeets/canjs,UXsree/canjs,thecountofzero/canjs,ackl/canjs,whitecolor/canjs,whitecolor/canjs,bitovi/canjs,airhadoken/canjs,asavoy/canjs,rjgotten/canjs,ackl/canjs,scorphus/canjs,dispatchrabbi/canjs