what is the process called where heat transfers from one object to another through an empty space, allowing fire to spread from one building to another?

The process is called radiation.

Heat transfer by radiation is when energy travels as electromagnetic waves (like the heat from the sun), so it does not need air, water, or solid contact to move. In a fire, intense radiant heat from flames in one building can cross open air, pass through windows or heat nearby surfaces, and ignite materials in the neighboring building, allowing the fire to “jump” the gap.