Minecraft wiki entry for The_Void, associated text: If using a Superflat map without bedrock layers, one can mine through the bottom and reach the void.
If using the "the void" preset, either mine through the starting platform or jump off the side of the platform.
Alternatively, the 'Bottomless Pit' preset for Superflat worlds will create a layer of cobblestone where the bedrock would be, which can easily be mined to reach the void.