No offense intended but that's the mark of a junior developer. Senior developers find themselves something to do, either to improve themselves or to help the business; preferably a single thing that does both. There are always new things to learn that will be a valuable career skill, there are always things at work that have been put off, like updating documentation, code maintenance, or adopting a new best practice. There is never nothing to do.

To forestall the inevitable complaint about overwork, you're perfectly free to do these things at as leisurely a pace as you prefer although you want to have accomplished something meaningful before the next batch of work comes in.