Answer
Storing "this key does not exist" in the cache, so repeated lookups for a missing item do not all reach the database, is called negative caching.
Also accepted: caching negative results, negative cache for negative caching.
Why
Without it, a client (or an attacker) asking for random missing ids sends every request straight to the database. Give negative entries a short TTL, so an item created later appears quickly; a Bloom filter of existing keys is another guard.