Liandry's Torment should have a Minimum damage on it's passive.

Liandry's is not mainly meant to finish people off, it's meant to chunk down tanks more effectively.

If you're looking for a buff, I would suggest increasing it's damage slightly based on maximum health while keeping the current health percentage damage it currently has.

Though the more practical solution that many others suggested is increasing the AP it gives from 50 to 60.

I've seen 0 damage ticks before, I don't think it does.