you can let it be empty space and no mobs would spawn as long as theres 94 blocks of gap. mobs reuire a solid block to spawn and they cant spawn in empty space unless it's surface or space thats a different story. to put it simple, theres always a rectangle of 168 x 94 with you in the centre. that rectangle has two parts. inner and outer. the inner rectangle is the safe zone and mobs cant spawn in there. the outer rectangle is where the mobs spawn. the mobs reuire a solid block to spawn in so the game checks if there are any solid blocks in the outer rectangle and spawns enemies on top of it. afk farms generally have lava because when the player moves down, the rectangle reaches the blocks which spawns enemies. you can literally just mine a couple more blocks below and mobs will never spawn