Return an empty relation instead of an array from db_relation()

db_relation() returns an empty array if it couldn't determine
another relation to use. This empty array doesn't work with
chained activerecord methods like find_each, and breaks
functions like populate_db_data_index!

app/models/manager_refresh/inventory_collection.rb

@@ -938,7 +938,7 @@ def db_relation(selection = nil, projection = nil)
                    rel
                  end
 
-      relation || []
+      relation || model_class.none
     end
 
     # Extracting references to a relation friendly format, or a format processable by a custom_db_finder