Pulling `:db/id` for a missing id gives different results than Datomic

[metasoarous]

In Datomic, if you pull :db/id for any eid x that doesn't exist, you get {:db/id x}. In DataScript, you get nil.

Further, (pull-many [:some/attr] missing-eids) evaluates to [] in DataScript, but (repeat (count missing-ids) nil) in Datomic (and of course, returns a vector of {:db/id x} when :db/id is pulled).

This bears some relation to #166.

Is this behavior you'd consider modifying to match that of Datomic?