Can I run a pressure washer without detergent in the tank?
If you're really worried you could fill the detergent tank with water. But the machines are designed with the knowledge that the detergent tank will run dry from time to time, or folks will forget to fill it... so if the manual explicitly says detergent is optional and there's no metering valve you can turn off (haven't used one, don't know) I would assume this is harmless.