Querying for a non-existing key for a state that has a default value set currently results in the default value being returned in the RocksDBStateBackend only. MemoryStateBackend or FsStateBackend will return null which results in an UnknownKeyOrNamespace exception.
Default values are now deprecated and will be removed eventually so we should not introduce them into the new queryable state API and thus adapt the RocksDBStateBackend accordingly.
This refers to
FLINK-5527 but solves the initial issues the opposite way.