They probably can.

NASA is worried about the thruster issue meaning that they lose control of the vehicle as it undocks and moves away from the ISS, leading to a collision. I guess that's independent of crew being on board.

But also ... the current Starliner software doesn't support an automated (uncrewed) undock. The previous one did, but some code and/or configuration changes are required to enable this on the current vehicle. NASA has said that making the changes to enable this will take about a month (including QA).