Minecraft wiki entry for Nether_Wart, associated text: Nether wart can generate in nether fortresses in soul sand gardens around stairwells. Nether wart can also generate in the courtyards of housing unit bastion remnants. Due to only spawning in these specific structures, it's entirely possible for a nether fortress or bastion remnant to generate without nether wart (though they may still appear in the chests that generate in the fortress).