Sorry, but a DT Rating of 0 is quite the same like no DT Rating. And reading the help Center of Groundspeak also helps:
Statistics that do not include Lab Cache finds, because Lab Caches do not include the appropriate attributes to be integrated into these statistics:
•Difficulty and Terrain
•Home Location Statistics
•Statistics on the Maps tab
Additionally, Lab Caches are not included in the My Finds Pocket Query (or other Pocket Queries) and will not result in any Souvenirs being awarded.