[router] Fix error in semtech decode which created empty structure when not needed
[router] Fix error in semtech decode which created empty structure when not needed

TheThingsNetwork/ttn,mgranberry/ttn,mgranberry/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,mgranberry/ttn,jvanmalder/ttn,LoRaWanSoFa/ttn,LoRaWanSoFa/ttn