get-document returns database status, not nil or empty map, if key is nil

[kenrestivo]

(get-document db nil)
;; returns
{:disk_format_version 5, :db_name "authtest", :doc_del_count 2, :disk_size 16473, :update_seq 4, :purge_seq 0, :compact_running false, :instance_start_time "1337823661155614", :doc_count 0}
;; and not nil

Some libraries depend upon get-document returning nil if the key is nil, so I ended up with this: kenrestivo/couch-session@145e5b7

[cemerick]

I think that prior behaviour of get-document is wrong. I've changed master to match it (nil or empty keys now return nil from get-document), but I think it should probably throw a NullPointerException…