If you have 0% of your masternode rewards going to an operator (meaning all the rewards go to the masternode owner), then you can use "" with the operatorPayoutAddress part. It will just be a placeholder then for the feeSourceAddress.
protx update_service proTxHash ipAndPort operatorKey
operatorPayoutAddress feeSourceAddress
protx update_service proTxHash ipAndPort operatorKey "" feeSourceAddress
Source :
https://docs.dash.org/en/stable/masternodes/maintenance.html#updating-masternode-information