A general term to describe all waves that can travel through empty space?

The only waves that are known and proven to travel through empty space are electromagnetic waves. Theoretical gravitational waves also travel through empty space but have not yet been proven to exist. Both electromagnetic and gravitational waves move at the speed of light, thus it could be argued that a general term to describe all waves that can travel through empty space are waves that move at the speed of light.

They say the message is the medium. In this case you could call it the "Ether". Ether is a blanket term describing information that is transmitted by any source through any medium. In this case the emptiness of space.