I entirely agree with your point with regard to a fixed-schema, row-based, relational-like datastore. But in a KV-store like DynamoDB, there's a different set of possibilities:

[1] key present, value undefined or null or some other special type

[2] key present, value is empty object of its expected type

[3] key absent

In schemaless datastores where the value's content somehow determines its datatype, it can be difficult to enforce a distinction between scenarios [1] and [2]. Meanwhile, in an externally-schema'd datastore (like most RDBMS), you don't have option [3]. I am familiar with the practice of mapping "we don't have a known-good value for this" to omitting the key in an KV-store, while in an RDBMS that semantic meaning is mapped to a NULL instead.