Focus on proper form and technique. It’s an easy way to get some quality pitching reps in even if you have no one to catch for you.
One of my personal favorite pitching drills to do inside is pitching with socks. I learned this from Bill Hillhouse.
Find a mirror or any other reflective surface. A glass door will do too. Roll up a pair of socks and practice your pitching by pitching the socks right at the mirror.
In my house, I happen to have a full-length mirror at the end of a hallway. The hallway floor is made of ceramic tile. Not only do the tiles serve as a method for measuring how far I am from the mirror, it also provides a built in power line.
If you don’t have a tiled floor, you can find other ways of laying down a power line to practice with. A piece of tape works well. If you’re lucky, you may even be allowed to put some tape on the mirror to create different pitching zone targets to pitch to.