If by some strange circumstance you’d completely traded out your 2019 draft hand and the picks remaining for 2020 weren’t enough to cover the deficit then it would carry over to 2021 I guess. They definitely wouldn’t let it go free.
My understanding is that a club wouldn't be allowed to match a bid in that situation. I believe there is a maximum deficit a club can rack up, equal to the points value of the rounds 1-4 picks that the premier will get, adjusted for any future picks already traded out or in.