I haven't been able to find a Sage Advice answering this question, though there is one about what happens when wild-shaped druids are targeted by Power Word: Kill. The druid reverts, but in that case, the wild shape ability explicitly says you revert "if you drop to 0 hit points, or die". This might be a hint that they don't drop to 0, since if they did, the "or die" section would be redundant.
Otherwise, looking at the rules text, it seems like the rules only specify how it works in the other direction:
When you drop to 0 hit points, you either die outright or fall Unconscious
However, the top of the Damage and Healing section has this text:
Hit points represent a combination of physical and mental durability, the will to live, and luck. Creatures with more hit points are more difficult to kill. Those with fewer hit points are more fragile.
A creature’s current hit points (usually just called hit points) can be any number from the creature’s hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.
It could be argued from this section that, since creatures have hit points, and the (newly-dead) corpse is no longer a creature, they no longer have a hit point total because they're dead.
Conversely, Revivify starts with the text:
You touch a creature that has died within the last minute.
which might mean that dead creatures are still considered creatures.
Ultimately, there isn't an explicit answer to this question, so the answer of how this works at your table is going to be up to the individual GM.