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Cooking Up Contingency Plans
(i.imgur.com)
Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
since there is no negative HP you are doing no damage past 0. So even if the spell hits it does no damage since there is no damage left to do, half of 0 is 0.
As I've pointed out in other comments there's a specific rule for damage past 0 HP. It's called “Damage at 0 Hit Points.” under the Death Saving Throws section, and it doesn't say anything about it getting cancelled out.