Each artifact contains a commitment. For the Product Backlog it is the Sprint Goal.
What should the Product Owner do when the Developers are ready to start the first Sprint, but the Product Backlog is not ready yet?
The "Definition of Ready" shows the team when Product Backlog items are ready to be selected for the Sprint Backlog.
Following Scrum, only what has already happened may be used for forward-looking decision making.
A Scrum Team member should take over both accountabilities Scrum Master and Product Owner.