Is there a block that represents 'void' in Minecraft?

Short Answer:

There is no void block.

Long Answer:

The void block does not exist simply because the void is just the point at which the world does not generate, and, as such, it is only air. The game itself detects you at a position at which it knows the void is in and deals damage accordingly.

Since 1.13 there are two new types of air: cave_air and void_air. Cave air is generated in most parts of caves and void air is used internally for some purpose for the regions below y=0 and above y=256. You can set both with commands, but they behave identical to regular air (except for /execute if block and similar of course). Void air does not deal damage, it is only the fact of being below y=-64 that deals damage.

The only thing that deals damage in the same way as the void is the command /kill. If you want to emulate something similar, the best you can do is to manually place void air or anything else and then let a command repeatedly give the player an instant damage effect whenever they are inside it. The cause of death will be different, but you can turn off death messages and print your own using /tellraw.