What should you do if the project manager is not assigning you any new work after the backlog already has been cleaned up?

I currently work as a junior front-end dev. For the past few weeks, the 2 juniors (incl. me) are lacking tickets. For example, I got 2.5 days of work assigned during our 3-week sprint. We already followed some courses, read some books & created some designs for upcoming features (that have no "functional specifications" written down).